When I had my 50th birthdays I planned a family weekend like this, staying at a lovely hotel run by some cousins, which happened to be fairly accessible for everyone I wanted to invite. I worked out that because it was special occasion I would pay for the dinner, and there would be rooms available if people wanted to stay overnight, at their own expense. It was a gamble, and I feared that very few people would actually come, but in the event they all came and we had a lovely time. My teenage son organised a surprise birthday cake for me with lots of sparklers, which was very heartwarming, even after my husband and I discovered he'd put it on our tab! After seeing this post, I'm still relieved that I chose to give this party myself rather than ask for money - it cost a fair bit, but I was more than recompensed by the knowledge that my family really wanted to celebrate with me even at some cost to themselves. Your poor sister-in-law won't have that pleasure, but other gransnetters' suggestion of taking her out for a special meal sounds like a good idea, and an honest and kind way out of a difficult situation. Might your wife's sisters do the same, do you think? The birthday girl would end up with a livelier social life overall. I do hope it can end up being a happy time for you all.