My son has the same. We had no problems until he got the swine flu and just didn't seem to get better from there. Was that the trigger that set the disease in motion for him, if not for that would he have never developed Crohn's, unfortunately we have no way of knowing.
I don't know if that changes the chances as even in identical twins it is 55% that they would both have Crohn's so I would think there has to be more to it then just genetics.
I still totally understand the worry as we went through it all before we had kids.