I'd tell them as soon as this holiday is over and you're all home, unless there's noises about booking it earlier in which case you need to mention it earlier. I wouldn't delay because the longer you put this kinda thing off the harder it is and before you know it, it's already been booked and paid for and you come off looking selfish (playing into their hands basically).
Say something like you prefer a different kind of holiday or to go at a different time of year or something. Just because dh is working in summer doesn't mean you can't get a holiday, look at ones more suitable to your whole family. Or indeed go away you and DC just but somewhere you like.
Personally, I'd try to find somewhere else to stay nearby that's cheap and that way you kibosh this crap but the kids still get their holiday (and you do too!) BUT I'm out of the FOG these days so tend not to take the crap - I well remember how hard it was before I got to that point.
I've experienced a similar dynamic in my case it got SO out of hand I was burnt and shoved and as a consequence went nc with my sister.
MummyofLittleDragon - I too have had to deal with the issue of my dd being scapegoat because she's my dd (and I was mum's scapegoat). My sister was the worst for it though and it was another reason for me going nc. At the point of going nc we were already on lc with sis and her kids because when my dd was around her kids could do no wrong (when it's just her kids she scapegoats one of them too - unfortunately they're not yet old enough to understand what's going on so when dd was around they were so happy to be out the firing line they joined in with the scapegoating).
Op - I agree - as time goes on I think you will need to be aware that your child/ren may be scapegoated by your mother and/or sister.
I'm wondering if pps who like me come from toxic families and already have done some work on this score or are good at recognising the dynamic are also noticing the posts likely either from pps who aren't familiar with the dynamic (and think it's easy to deal with these people) or are on the other side of it (and don't understand how it feels to be on op's side of things).
It's easy to say 'confront' 'challenge' when you haven't had to deal with the kind of people likely to turn it into a whole drama where the op's 'guilty' for 'ruining the whole holiday' AND carping on about it for DECADES!
Just read update at 1208 today - fuck em all off! Appalling way to behave your father hasn't even been there so only has what your mother and sister have been bitching to him about! So they've no qualms about spoiling his time with his family and he's no qualms about believing them without even observing properly or listening to your side! What are they like when you're at home? When they visit yours and vice versa or days out? (Think I can guess!)
Even IF (and I genuinely don't believe it to be the case) you were a lazy type they've went about dealing with it in completely the wrong way, IF someone wasn't pulling their weight, you sort a rota or directly ask them to do X - you don't take over and then bitch about it to everyone else!!
I don't think op's dh is unsupportive as such, just probably unfamiliar with this dynamic from a NORMAL family himself and frustrated at as he sees it, op complaining but doing nothing to change things (which is kinda true but also more complicated due to FOG and women being socialised to please all the time - especially family - men don't generally put up with this kinda crap cos they're generally socialised to expect to be the one being pleased not the one doing the pleasing - excepting those also raised in toxic families of course). My ex was the same. On one occasion with my sister I was at end of my tether and asked him to step in on my behalf and he actually did so very well - and my sister was on best behaviour for several months after.
Op what does your dh think you should do?
"Right so I’ve told my DH and he wants me to confront them and says I should go home with him this evening." He's right.
I also think it could become VERY interesting if these 2 toxic idiots are left with only each other to scapegoat!
"Mum while I appreciate you paid for the cottage I'm not enjoying this holiday because you're manipulating the chores to make it look as if I'm being lazy AND bitching about it not only to each other but dad too" turn to face dad "and you didn't even think to get my side of things first" to all "I am looking after 2 small children whereas the TWO of you only really have responsibility for one child, my parenting is also being undermined repeatedly and it's negatively affecting both my DC's behaviour too. So I'm going home with dh and I won't be holidaying with you in future" don't get dragged into a long argument about it all. Pack beforehand and be ready to just go.