I dont know the background to this (and it seems there is a lot!), but OP you are clearly known on here, so I suppose what I am asking is what do you want to gain from posting? Is it advice? Is it just to rant? Is it to see if someone is in the same position as you? I'm sorry if I've missed this but I can't work it out.
I'm trying to work out what gone on, but I would say that there is nothing wrong with the holiday without your DSC especially if your mom has paid for it BUT that would depend on their general relationship. I also do think you should have cut the holiday to ensure he didn t miss contact time, surely it could have been 10 days? It seems as if your DP and his DD's have a really poor relationship- that's the first thing that needs to change, he needs to be consistent and spend 1-2-1 time with them, could they have a short break together? Or a couple of days out alone? Could you have 1 weekend were he just sees them and the next you all go?
In terms of their behaviour, we all now kids can behave awfully at times, even your own child said something that's nasty and upsetting. Maybe a bit of sympathy and understanding would help them behave better?
With the sleeping arrangements, I really think you need to rethink this, could you go in with your son and your OH shares with his DC (I'm not sure of their ages?). I would also start being really positive about his DC in front of your son- you may not have meant it but it clear your son has picked up on your feelings towards them. So what if they have nits, all kids have had them at some point, use the shampoo to treat the kids, imagine how uncomfortable it is for them.
Maybe you need to step back slightly and consider that they are all children and do not have the emotional maturity to manage their feelings, so as adults we need to try to understand why they are acting in a particular way, if it is just bad behaviour there needs to be discipline with the same rules for all the children.
It sounds as if you have had some bad luck (eviction, poorly child) but his DC are not to blame. If you feel stressed by it, imagine how they feel they have no control to change the situation. Sometimes money pressures can make us behaviour unusual, or cause us to resent the wrong people- it's up to you to recognise that and change the way you approach things. If I'm honest there's seems to be a lot to blaming going on, rather than stopping and taking some responsibility for some of the situation.
Like I said if you could be clearer why you are posting it may help.