I am now the mum of an almost 12yo and a 14yo.
Your post was me 13 ago.
Relations with my ILs have always been strained - they decided I was no good from our dating days and I have no real idea why. They have been vile to me right up until about 3 years ago. Told me I would not look nice on our wedding day etc etc.
MIL was when the DC were small very very controlling over them and would lie about things and mess with my rules just because she wanted to prove she could. Just basic silly things like my 3yo having a tantrum about eating one of her easter eggs one easter day literally as I went to leave the lounge and go to the kicthen to dish up a roast easter dinner I had cooked for the family. DD had already had more chocolate that morning than usual so I said no to DD but that if she ate her dinner she could have some chocolate that afternoon. but no the minute I left the room MIL was stuffing smarties aand chocolate buttons into DD.
Anyway DD1 was 1 and we were co-erced into a family holiday to Greece. I didnt want to but our compromise was we would go for 2 weeks and they would come for 1 week - the 1st week. It was a bloody disaster. They did not offer any child care once (fine to a point but that was one reason they wanted to holiday with us - to help us with DD1 and give us some alone time). It didnt worry us that much we had a beautiful sea view so would sit with a bottle of wine and watch the sun set playing cards or scrabble whilst DD went to bed. But then every night they would come upto our terrace on their way back from the entertainment and rave loudly about, tell us what a fab time they had had and wake DD up in the process.
We took the bus and DD off out for the odd day trip (to give us some space from them) whilst they stayed at the hotel but they were like a pair of little kids and MIL had a tantrum because she had been bored all day with FIL and had wanted to come to X town - forcing DH to take her homself again the next day.
Sorry I am waffling on - but we tried all the compromises. Split accommodation, days out etc etc but they still dominated our holiday. It stretched us financially and we had a pretty poor and difficult week the first week because of needing to put up with their selfish behaviour. MIL made difficult and unneccessary comments all week. I bit my tongue but even DH at the end of it could not wait to wave them off to the airport and our 2nd week was bloody lovely - just me DH and DD1.
If your relationship with MIL is shit - dont go. You wont enjoy it and if she is still on a power trip or feeling threatend by your motherhood or whatever - it will be shit.
Make out you cant get leave the weeks they are free to go and maybe arrange a short break in the UK with them instead- maybe a Fri to Mon one. This will serve 2 purposes. Hopefully pacify MIl to a certain degree at the dissapointment of her holiday not going ahead and it will give you and DH (and your ILS) to suss out how everyone will behave on a break away. If it goes tits up - its only a few days to grit your teeth and not too difficult to pack up and come home and dissapointing that will be, its a weekend away and not your main holiday of the year ruined.
Good luck