Wow, that is a very tough situation. I am so sorry.
If I were in your situation I would send him a STD, only because he is your FI's brother. I know he is acting like an immature jerk, but there might be a lot of reasons why he is acting this way. Perhaps he is jealous, maybe he wants to go but his wife doesn't and they got into a fight about it, etc...there could be a million reasons. This of course doesn't make it right, but by sending their family a STD you and your FI are acting like the better person.
I hope this helps and good luck!