I do t think you are awful OP, it sounds like you are struggling and I think you are very resentful of the situation you are in.
What does your DH say about your feelings??
How is your relationship with your DH aside from his oldest DS.?
How is he with your DS when his other DS arrives ?
Sounds like maybe you need to take a little step back and start again. The ex wasn't obviously very understanding in a very upsetting time for you, but your DH should have managed that properly and said he needed to be his poorly baby, the same as he would go to his older son if he was poorly.
You should have gone to Lanzarote anyway - DSS gets invited, if their mum refuses them to go, she is the one stopping access so it's not unreasonable to have a holiday. Just go.
Doesn't sound like his mum spends much time with him at all, if you have him every weekend.
Why is it every weekend, is this court ordered or choice of your husband - can he do 3 out of 4 weekends maybe?
Your DHs son does just sound like an 8 year old who would wind a sibling up and annoy them. I wouldn't have him shouting at my 3 year old though, no one shouts in our house, I don't care who you are, it's just not done and if it does, fun stuff is taken away.
You need to have no contact with DH ex wife, that's his job. It's his child and his ex wife, not for you to deal with organising. If DH is working their is no reason for his son to visit if you two are not close.
We know a family where they lied to the kids mum that the dad was at home when he was actually working all day, but the mum wouldn't let the kid come if he was only coming to spend most his time with the dads wife. I would (hypothetically) be livid if I was loosing out on time with my Dc to my ( hypothetical) husbands wife.
The basics are, if you can sort the issues you have with your DHs oldest son (and they are you and your issues with your DH, he's only 8), is your relationship worth saving or is it dead in the water anyway.