No way are you being unreasonable ....but how to pursuade your wife otherwise ? even if you can easily afford it I think your sister in law expecting everyone to cough up 500 pounds is selfish and greedy ...... but if you really , or your wife, really, wants to go to the party try and find a travel lodge nearby ...that way, you get to go to the celebration and your s-i-l may not get the ten people she needs to get her freebie ,,,if you know any other guests you could suggest that to them as well ...or you could just reply to her invitation saying sorry cannot possibly afford that kind of sum ..have a happy birthday ! if enough of the guests cannot afford it maybe she will have to think it out again !