1) This is your and FI big day - if they come / or don't then don't let it spoil it
2) Take it you are a few months pregnant- no one has a right to tell you to abort. He should keep his opinions to himself, it’s nothing to do with him. It an additional stress you should not have to deal with
3) Sitting down at talking to them is an perfect answer but this doesn't always work. If you follow this advice, take care not to let it escalate into a full blown row. Usually this method requires maturity.
4) You, your FI and babies are a beautiful family. Remember that saying about not letting anyone part you?
5) If all else fails remember we all have some of those embarrassing family members we cringe at!