I don't think you're being selfish! It all comes down to what YOU and FI want! Will he regret if his family isn't there? Maybe....but only he can decide. I'm going to be brutally honest from my experience: Even if you give in and cater to them, they will find something else to disagree on (ie the time of year, the price, the resort, you name it!!). I catered to people for my wedding and several of those people didn't have the courtesy to RSVP!! One of my important family members that I did not cater to, made the necessary arrangements and hasn't complained once. I think that the people that want to be at your wedding will, and for those who don't.....well that should be a message to your FI. If his sister is honestly that spoiled that she only wants to attend if its somewhere that she wants to go, then I would let her miss out. In all honesty, who would regret that more? She should!!!
Anyways, I would just hate for you to give in and then have it snowball to the point that your guests or potential guests are running the show, KWIM? I think you should sit with FI and decide what you both want and run with it! It's your day! Good luck!