Ugh, honestly I feel your pain...I'm having troubles with my sister too, but that's a whole other story lol! Honestly, I would sit down with your aunt and uncle and tell them how you feel, and how them talking negatively to other family members is making the rest of the family feel. You can go about it in a way that is friendly and not to hurt their feelings. Like just say something like, we appreciate your input, but this is what we have decided to do and we are not changing the plans, and we hope that you can come but if not we will respect that (kinda already what you said to them, but make it clear that this is your decision and it is final). But really, this is your and your FI day, and you should be able to plan it however and whenever you want to. Its fine for people to give their two cents but then they should be polite and respectful, and drop it! Ugh! It really gets my goat when family tries to chime in all the time and tell couples how things should be done... I'm lucky that neither of our families are really like that Good luck!!