Question: If my twin had breast cancer, does that mean that I will get breast cancer too?
Answer:There are two different kinds of twins. There are twins that are monozygotic, or identical twins, meaning that they came from the same egg, the same fertilized egg, which means that they share, essentially, all of their genetic material. And then there's fraternal twins, or dizygotic twins, that come from two different eggs.
So identical twins, if their breast cancer is due to an inherited predisposition in a mutation in one of these genes that's been identified, then they would be at significantly increased risk -- but not predetermined or destined to have breast cancer. Even people who have an inherited mutation in one of these breast/ovarian cancer syndrome genes do not all develop breast cancer. So not everyone would.
As far as the fraternal twins, they should be no more likely necessarily than other siblings, who share half of their genetic make up. However, twins share some more environmental factors than maybe siblings that grew up in the same household, but did not share the same pregnancy or the same environment even in utero.
So it's a difficult question to answer and it's one that's actually been studied pretty well to try and help clarify how much of breast cancer risk is genetic and how much may be environmental. It's an interesting question.