The original rule of Cosa Nostra was that made members had to be 100% Sicilian. Naturally, I suppose, as it is the Sicilian Mafia. That being said, when the Moustached Petes consolidated power in New York during the early 20th century, masterminds like Lucky Luciano realized they could make far more money, gain far more power and influence, and really run a far more vast enterprise by doing business with non-Sicilians. Luciano was good friends with Meyer Lansky of course and many Jewish mobsters were close business associates with LCN members. Also during this time, the books were opened up to Italian-Americans, even if their ancestry was traced back to the mainland rather than Sicily. It was a controversial move. The mobsters back home didn't like this and some in the United States like Vito Genovese looked down upon working with men who were not Italian (although he himself was somewhat of an outsider in the organization as a Neapolitan). By mid-century, La Cosa Nostra members were instructed to "keep it within the family" and members married relatives of other members to ensure their blood was "pure." Associates could still be of other ethnicities (but always of European descent ) however nobody powerful would be a non-Italian. This became the norm through probably the seventies or the eighties when the mob was in disarray. At this point the rules were bent to the point where any man whose father was Italian could be initiated into the organization. Notable examples of this include the aforementioned "Cadillac Frank" Selemme whose mother was Irish, John Junior Gotti whose mother was a Russian Jew and Tommy Del whose mother was Polish. The former two were acting bosses of their respective families which would've been unheard of during the early days. They would've been whacked along with whoever sponsored and made them by the Sicilians the second their ancestry was discovered.

This rule remains more complicated to this day. It truly depends on the family. Some are stricter than others when it comes to "purity." These days its hard to find someone who is 100% Italian anyway. La Cosa Nostra could not realistically remain a force to be reckoned with had it not been for this rule change, however the most widely accepted agreement I've seen is that a person's father has to be Italian.

Now there have been a few examples of men being made who had zero Italian ancestry. John Veasey was made into the Philadelphia Crime Family by John Stanfa despite being a non-Italian. Vito Rizzuto was notoriously the most lenient in regards to ethnicity when he was the boss of Montreal. He has made members into his family who were not Italian at all. It is almost unfathamable, but the fact remains Juan Ramon Fernandez, a Spaniard, and Raynald Desjardins, a French Canadian, were made members of La Cosa Nostra and this caused quite a bit of friction between Rizzuto and the Sicilians. Fernandez was actually murdered in Italy.

So the question of whether or not someone has to be 100% Italian to be made is a complicated one, but the short answer is "no."