Honestly, if it were me, I'd tell people we've been trying and it hasn't happened for us yet. That'll shut them right up and might get round to other people before they ask you. I understand you probably have different comfort levels on what you want to share, that's just how I'd approach it.

Also, you do not have to go. You could talk to your SIL privately and explain and then make up some excuse for everyone else, business trip, ill health, already booked holiday etc.

Your SIL, I assume, has more than you and your partner to lean on so do not put too much pressure on yourself to support her.

Your mental health is first and foremost, and while that may seem selfish, it's the best way to be. It's the oxygen mask; you can't help others before you help yourself.