The Catholic Church teaches that The Virgin Mary was indeed without sin. It is a fairly bizarre dogma which goes something like this:

Adam and Eve were created without sin on their souls. Because of their disobedience to God (letting a talking serpent tempt them into eating a piece of fruit from a magic tree),they committed what is known as the Original Sin.

The result of this is that even if a person managed to live their entire life without sinning,they would still have the stain of Original Sin on their souls,since it was passed down from Adam and Eve. Therefore,no one could be entirely sin-free.

And then along came Mary.(apologies to the Association).

As the story goes,when God decided to have a kid,there were several criteria that the surrogate mother had to meet.
One of these was that she be a virgin,and the other was that her soul did not have the stain of Original Sin on it.

The solution to the Original Sin thing was another bit of cosmic smoke and mirrors called the Immaculate Conception.
Contrary to popular belief,this did not refer to Mary conceiving Jesus without having had sex.
What is taught by the Church is that when Mary's mother conceived her,God removed the Original Sin from her soul,therefore it was MARY who was conceived immaculately,allowing her to be the only human in history who lived her whole life totally sin-free.

A few centuries later the Church had to do a little tap dance on the Mary saga,after someone pointed out that since "the wages of sin is death",(according to the Bible),Mary couldn't die,because she never sinned. If she couldn't die,then why wasn't she seen around town,(going to weddings,doing her shopping) for the next thousand years or so?
The solution to this little dilemma was, even by Church standards, a real piece of work.

Bada Bing,Bada Boom,the Church comes up with another gem called The Assumption of Mary.
According to this theory,Mary was bodily assumed up into Heaven
while still alive. One minute,she is hanging out with her lady friends,maybe having some figs,or getting her nails done,and next thing ya know,she starts floating up and up,eventually reaching the Pearly Gates. By the way,this is the actual depiction in Church literature,she literally is shown in old artwork as ascending in the air.

So,for what it's worth,that's the way the story goes.