Crohn's is a bit odd genetically. While having a parent with Crohn's increases your chances by 2-3x's, having a sibling with Crohn's increases your chances 20-30x's. So, it's not surprising that your wife and her sister as well as your dad and his sister have it. There's a strong correlation in siblings. There are so many genes associated with Crohn's, though, it's hard to pin it down.

I was diagnosed with Crohn's seven years ago at the age of 28 and both of my siblings have started to develop similar symptoms over the past couple of years.