Oh shes aware its not going to be a regular thing. And to be fair it's out of character for her. Shes normally as far as I know or in my experience happiest in our own company. Not having to make small talk while on beach or sunbathing etc
She's made a grave error but I think you both already realise that!
Look, try to put your foot down about MIL coming in with you - tell her straight that you don't want her in your bedroom and that she needs to sort it out, but otherwise I think you'll have to pretty much grin and bear it.
From experience, to help you get through it, what we did when we realised that our couple holiday became an entourage of 15 people, we quietly rebooked our own room in a totally different hotel on the other side of town, said nothing and pretended that the hotel contacted us very last minute due to an overbooking and put us in their sister hotel. Such a shame.
Then we booked loads of day trips -again saying nothing. We did waterparks and scuba diving and snorkelling... it was such a shame that the ringleader gets terribly seasick and couldn't come.
We did meet up for I think about 3 evenings in all, and one day with some of the nicer ones on a boat trip so it worked out as best it could really - from then on it was an emphatic NO if anyone even hinted at joining us on holiday - I just tell them straight that my holiday time is family time and DS and DH are the only ones allowed on it. I'll take trips separately with others like a weekend away leaving them at home, but no group holidays ever. Not even if totally free. You just won't get me on a fucking plane.