I agree, it is definately FI's responsibility to talk to them. You don't want to start a fight and have them blaming you. If FI is close with his dad, maybe he should start by talking to him. Tell them with all of the wedding expenses, you can afford to help them out a little, but cannot afford to pay for the whole trip.
Your sister might still help you out with the photographer. Maybe it could be her wedding gift to you??
Good luck. I have a hard time dealing with my FMIL and FFIL as well, I know how hard it can make life.