Firstly, rid yourself of any possible "you knew what you were getting yourself into" utter shite.
This is very little to do with a blended family, and a lot to do with the individuals concerned, irrespective of who each individual's biological parent is.
You have an adult DS who has failing mental health, but won't seek help. There is nothing you can do. I'm by no means saying wash your hands of the guy, but realistically, you've got as much influence on his medical situation, as you have your next door neighbour. So ask yourself, ask DH, how much of your life will be spent on trying to assist and adult who refuses to be assisted. It's shit. But you can spend the next ten years running yourself into the ground, to achieve the same result as if you had resigned that he will make his own decisions, now.
Your exH is too preoccupied with his new younger family, has little time for eldest DD, and her plans are continually upended. She's fed up with him. Stop sending her. Stop making her available at the last moment because he's too busy with his priorities and she has to be crammed in at his convenience as an after thought. Let him sort that out. It's not for you both to live your lives around his last minute plans. Children need structure. Plans are made a fortnight in advance from now on.
Your real issue is that fact that (and I can't tell which is the more driving factor here) you and DD don't want his boys at the house, or the boys don't want to come. This is your big problem, and why you have no time with DH...he literally moves out for chunks of time because his children don't come to his home.
"It's a bit cramped" doesn't really cut it. It sounds like you and DD don't particularly enjoy them being there, and so they've kind of been phased out of their dad's home.
So, either, you and your daughter get over the fact he has children who equally need to feel welcome and be accommodated in their parent's home, or carry on with them "at bay" and you can see where this is heading...