Without wanting to write too much of an essay...
MiL had a really hard time earlier in the year and felt very low. She and FiL wanted to plan something to help her feel better/ give her happy memories. They decided they wanted to take us (their son/my DH, me and our two children), plus their daughter (my SiL), her two children and SiL's new partner away on a holiday abroad, which is obviously very generous of them. Challenge now is that SiL and her partner are insisting that his two children must come as well.
Relevant info here... 'new partner' is SiL's boss. They had an affair in work for a year, my BiL and boss' wife had no idea. We only found out at the end of 2024 when they did. (None of us saw it coming and are all still feeling upset as everyone was close to SiL's husband and understandably don't agree with the affair). My PiLs and my DH/ children have only met new partner's children once to date. It didn't go brilliantly as my SiL literally just brought them to PiL's house with no discussion beforehand which upset my MiL, who didn't feel ready to meet them. Also unfortunately during the visit one of our kids felt picked on by one of his kids. (I did overhear his child saying some unkind things to ours and our child was definitely left out of play by all the other children which was naturally upsetting for them).
So PiLs don't know what to do - we've all said that we'd rather not have his kids come as we barely know them and it changes the whole dynamic of the holiday. SiL has already said they'll pay for his kids to come so can't use that as a barrier. She is saying if they can't come then she and her children won't be either.
I can see her point in that it might be hard for his children if they knew their dad was going away on a holiday and they weren't invited. SiL has also insisted that new parter comes. They all live together 50/50 so his kids would have an awareness of my SiL's children having a holiday too.
Equally I feel protective of my own children's wellbeing and them getting time with their cousins. We don't get to see them that often so time is precious. I'm worried that we'll go on this special holiday and our child will be picked on again and it will be miserable for them (and subsequently miserable for the rest of us).
MiL and I have shared that it does feel a bit like we're all being forced by SiL to accept her new blended family really quickly, when none of us have had time to really process it all. (Within two weeks of SiL telling her children that their parents' marriage was ending she introduced them to her new partner and his kids and they moved in with him less than a month later, so from the outside this has all felt a bit rushed to the rest of us).
MiL feels trapped as she doesn't want to cause drama (SiL will react strongly if told no) but equally feels strongly about this trip being planned as her being able to spend time with her grandchildren. I guess my AIBU is given I'm being asked my opinion by MiL am I unreasonable to be honest and voice that I'd prefer the holiday to not include new partner's children? (Even though I fully respect it's not my decision as I'm not paying for it).