it looks tp me, like you are just having a good vent on here. I hope thats all it is. Step children can be a difficult thing to accept, and understand, and embrace into your family life. However, they are part of your family, and were there before your two DC came along. You knew they were part of the package when you married your DH surely?
Its abit late in the day to now be resenting his other children. I cant help thinking that maybe this is something you need to discuss with your DH. I mean there has to be give and take. I understand him wanting to see alot of them, of course he does, they are his children. HOwever, if it is making you so unhappy, then i think he also needs to know this, and hear what you are saying, and come to some kind of compromise. Especially, given that their own mum wants them to be home more too...
I think its going to be a long chat around the kitchen table. If you are so unhappy, then his youngest DC are going to suffer ultimately, as will your relationship with him.
All that said though, I'm afraid I do think you are being a little unfair on his children. Whats wrong with asking what you are doing? or, talking to anothe new mum when you had a new baby, your step child was just taking interest surely?
anyway... not having a go at you here, in anyway, so please dont take it that way, just pointing out a few things. I do hope you get something sorted out with your DH, you sound exhausted TBH.