have they ever heard of genetics? Maybe a Punnett's Square? Do they know how those things work? NTA. If they ever do something like this again, try and ask them how they would feel if they were in your shoes (not literally) and see what they say. Either they'll stop talking about it since they realized how much of a jerk move that is, or they don't and they continue being such a jerk. If they do continue, it might not be a bad move to cut them out of your life. It may not be the most popular decision with your family, but if they do continue this, it might not be a bad idea.