Are most cancers genetic?

My wife and I have alot of cancer on both sides of the family. Hers – MIL with breast cancer, uncle with testicular cancer, grandfather with pancreatic, half sister with Hodkin's. She had cervical cancer but it was taken care of fast and she's fine now. Mine – dad with stomach cancer, and now my aunt has skin cancer. What of this is genetic? We have a 5 and 3 year old and are expecting a boy in May. I'm just scared with our lousy genes our kids are going to get sick.