I'm wondering if there's a definite timeline for the first two GF II movies?

As I've seen it:

1901: Antonio Andolini, his wife, and son Paolo, are all murdered in succession either by or on the orders of local gangland Don Ciccio in Corleone, Sicily. Vito Andolini, the younger son of Antonio, narrowly escapes his own murder and flees to America, and there is renamed Vito Corleone.

1917: By this time, Vito is married to Carmela, and has a son named Santino. He loses his job working in the shop of friend Genco Abbadando father due to local neighborhood boss Don Fanucci, and becomes involved in organized crime through Peter Clemenza to make extra money. He witnesses Fanucci menacing a young woman around this time.

1920: Vito and Carmela have had two more sons, Fredo and Michael. Fredo is a sickly boy; Michael is an infant born earlier that year. Vito's status as a rising star in the underworld attracts the attention of Fanucci, who attempts to extort and intimidate him. His partners, Clemenza and Salvatore Tessio, wish to avoid trouble by paying Fanucci in full, but Vito insists that he can convince Fanucci to accept a smaller payment by making him "an offer he won't refuse". During the Feast of San Rocco in August 1920, he stalks Fanucci to his apartment and shoots him dead and becomes the new neighborhood Boss.

Mid 1920s: Vito, now a respected and rising Mafia Boss, opens Genco Pura Olive Oil Company with his criminal associates as partners, as a front for his illegal activities. It is around this time he partners with a young hood named Hyman Roth to import illegal liquor during the height of Prohibition, further increasing his power, wealth and prestige.

1922: Vito takes his family, which now includes a daughter named Constanzia, on a trip to Sicily. Ostensibly, this is a family vacation; Vito however, intends to not only expand his empire overseas, but also settle old scores. With the help of a defector from Don Ciccio's camp named Tomassino, he avenges his mother, father, and brother by stabbing Ciccio to death. In the fight, Tomassino is wounded and crippled. However, the two remain lifelong friends and business partners, allowing Genco Olive Oil to become a multinational business; at the same time, Tomassino takes over Ciccio's territory and becomes a trusted and loyal Corleone Family ally.

1932-1933: A war involving Al Capone, Maranzano, and other rival gangsters results in the wounding of Vito. New Corleone muscle, Luca Brasi, helps end the war by murdering Vito's enemies. The war is settled with the murder of Maranzano, his empire absorbed into Vito's own, and Vito organizes the Commission, to help regulate La Cosa Nostra. He is at this time the most powerful gangster in America.

Sal Tessio, and Pete Clemenza are his Capos; Genco is his Consigliere; and Frank Pentangelli is one of his higher level Soldieri. Vito adopts orphan Tom Hagen into the Family, raising the boy as his own son, at the request of Santino. Santino begins to join the family business at this point.

December 7th 1941: Michael Corleone enlists in the Marines on Vito's birthday, much to the disappointment of his family. Santino introduces his sister Connie to a friend and associate of his named Carlo Rizzi.

Early 1940s - Michael meets and begins dating Kay Adams while attending Dartmouth College.

1944-1945: Genco is diagnosed with cancer and his health fails. Hagen is made interim Consigliere, much to the disdain of the other NY Families. By this point, Santino is acting as Underboss of the Family.

August 1945: Michael returns from military service to attend his sister Connie's wedding to Carlo Rizzi. By this time, Willie Cicci and Paulie Gatto are Enforcers in the family under Clemenza.

September 1945: Virgil Solozzo, a "young Turk" gangster and heroin producer, obstenstively backed by the rival Tattaglia Family, meets with Vito and the Administration of the Corleone Family hoping to establish a business relationship wherein the Corleone Family will help Solozzo distribute heroin and offer their political protection over his operations in return for control of the business and a large chunk of the profits. Santino openly expresses interest in the deal, much to Vito's disapproval; and Vito ultimately turns Solozzo down, seeing the drug business as potentially a threat to his Family's own businesses. It isn't known yet, but Emilio Barzini, a rival of Vito's and Boss of the second most powerful crime Family in the country, is secretly backing Solozzo and using him as a front for his own interests. Smelling a rat, Vito sends Luca Brazi to act as a spy on the Tattaglia people under the pretext that he is unhappy with Vito's leadership.

December 1945: Luca Brasi is murdered by Bruno Tattaglia. Vito Corleone, on the orders of Barzini, betrayed by Paulie Gatto, is ambushed and nearly assassinated. He lingers in critical condition. At the same time, Tom Hagen is taken hostage by Virgil Solozzo to attempt to force the Family into cutting a deal. Vito survives, much to the chagrin of Solozzo and Barzini.

Sonny becomes Acting Boss of the
Family.

Clemenza protege Rocco Lampone is inducted into the family after murdering Family traitor Paulie Gatto.

Michael joins the Family after his jaw is busted by police Captain Mark McClusky, who, acting on the orders of Solozzo, whose payroll he is on, removes security from Vito's hospital room - in the hopes of pulling off a second, successful assassination attempt.

January 1946: Michael assassinates McClusky and Solozzo for the Family, and is sent into hiding in Sicily. Fredo is sent to Vegas under the protection of one of the Family's Western allies.

1947-1948: The War between the Tattaglia and Corleone Family drags on, the Corleone Family bumping off Tattaglia second-in-command Bruno (son of Boss Philip Tattaglia) while Vito slowly recuperates. Enforcer Willie Cicci is bumped up to Soldieri during this time. The power of both Families declines as the War allows for both the Barzini Family and non-Italian gangs to grab hold of their gambling rackets. The War is locked in stalemate. The police crack down on the underworld's vice rackets.

1947-1948: While in Sicily, Michael meets and falls in love with Appolonia Vitelli, and courts her. They are married a short time after.

1948: Hoping to end the War, and cripple the Corleone Family permanently, the Barzini Family, with the assistance of turncoat Carlo Rizzi (embittered by being beat up by Sonny and being kept out of the Family's inner circle), set up and brutally gun down Sonny.

Vito rises out his sick bed and takes back control of the Family, and organizes a peace summit. Meanwhile, in Sicily, a botched assassination attempt by the Barzini Family on Michael results in the car bomb death of his wife, Appolonia.

Vito and Barzini organize a summit which include all of nationwide Bosses of La Cosa Nostra. Barzini presides over this summit, ostensibly as the neutral peacemaker, which only serves to enhance his status as the new leader of the Underworld. A peace deal between the Tattaglia and Corleone Families is reached, in which the Corleone Family will lend its political power to support and cover the nation-wide traffic in Heroin for the other Families, in exchange for a return to peace and resumption of the status quo. Vito hinges his deal on the clear command that his son Michael be allowed to return safely to the US.

1950: Michael returns to the US, his name cleared in the Solozzo and McClusky murders, and he begins to be trained by his father to take over the Family, and he is introduced to his father's many political contacts.

Father and son begin to devise revenge on Barzini, Tattaglia and their supporters, and also begin to develop the plan to relocate the Family to Nevada. Outwardly, the Family will appear weak, while secretly creating new regimes.

Around this time, ex-policeman Al Neri is cultivated to replace Luca Brazi, and serves as Michael's personal Soldier and chief enforcer along with Rocco Lampone.

Fall 1951: Michael reunites with Kay Adams, and promises her that the Corleone Family will be "completely legitimate" within the next five years. They are married not long after.

1952: Michael and Kay have their first child, Anthony.

1953: Michael and Kay have a daughter, Mary.

1954: By now, plans to move to Nevada within the next year are cemented.

Vito retires as Don of the Corleone Family and semi-retires, acting as the Family's de Facto Consigliere, while Tom Hagen is removed as Consigeliere and also from the criminal end of the Family. He remains the Family lawyer. Michael becomes Acting Boss of the Family.

Meanwhile, the Family's seemingly passive stance has allowed the Barzini people to chisel away at Family Territory, and Capo Salvatore Tessio and Pete Clemenza chafe under Michael's leadership. Tessio is the more notably distressed of the two. They are ordered simply to be patient.

Michael meets with Moe Greene, the head of the Las Vegas underworld and an important underworld figure, who has been instructing and also belittling and abusing Fredo. Michael attempts to buy Greene out of the shares of his hotels and casinos. Greene rebuffs Michael, saying the Corleone Family is finished.

Spring 1955: Michael continues to be instructed by Vito, who warns him that Barzini will move against him soon, and that there is a traitor in the Family. Vito predicts that this traitor will set up Michael's murder by acting as a mediator in the Cold War between Barzini and Michael, setting up a meeting between the two and guarantee ring Michael's safety.

June 1955: Vito dies of a heart attack, aged 63, while playing with his grandson, Anthony.

At his funeral, Tessio reveals himself as the Family traitor by informing Michael that he is going to set up a meeting between Barzini and Michael at his territory in Brooklyn, promising Michael's safety.

July 1955 - Michael serves as Godfather to Connie's son, Michael. The same morning, on his orders, Emilio Barzini, Philip Tattaglia, Carmine Cuneo (Boss of the Bronx based Cuneo Family, the third most powerful in the nation), Victor Stracci (Boss of the Staten Island based Family bearing his name), and Moe Greene are all simultaneously assassinated.

Al Neri kills Barzini; Rocco Lampone kills Tattaglia; Willie Cicci kills Cuneo; and Pete Clemenza kills Stracci.

Later that morning, Sal Tessio is taken to be executed by Willie Cicci, among others, under the supervision of Tom Hagen.

That afternoon, after confessing to Michael that he set up Sonny to be killed, Carlo Rizzi is executed by Pete Clemenza.

It is likely Tessio's territory is split between Al Neri and Rocco Lampone, who are elevated to Caporegime. Control over Moe Greene's casinos is absorbed by the Family, as is much of the territory of the other Five Families.

The Corleone Family is now the most powerful in the country, if not the world.

August 1955: The sale of the properties in New York, and the move to Nevada are almost complete. Michael allows Clemenza to retain his position as Capo and act as the family's representative in New York after the move, while Neri and Lampone will act as the Family's Capos in Nevada.

Connie hysterically confronts Michael and accuses him of murdering Carlo, which he denies both to her and to Kay. Michael is officially made Boss of the Corleone Family, his ascension officially approved by Family Capos Clemenza, Lampone and Neri.