She's got them saying they don't want to see you or speak to you, right? I think you should still pop in the conversation and just say hi.  He does need to stop cowering and doing whatever BM wants.

I would just let it go. If you let BM know her antics bother you or DH she will only ramp up her game more. Inevitably causing the children to be in the middle. If you really care about them take the stance of the bigger person who won't play those games.

After a while if BM thinks you don't care about the kids she loses using them as pawns for her own satisfaction. YSD is smart and we both keep anything we do between us. Also we have grown a lot closer because both parents worship the ground OSD walks on and yell at her when she says anything negative about OSD even though it's 100% the truth.

So she has learned she can come to me to vent her feelings and it stays between us. 

I hate that you hate yourself for having completely normal thoughts! Of course you would feel hurt. The BM is trying to cut off your relationship with your boyfriend's kids, and he may be going along with it.

Well, he hasn't gone along with it yet, so i would say wait and see what happens. But, if it does, you are of course justified in being upset. It's not a good sign that he would allow her to control him to the point that you can't even say hello to his children. It's a sign that a future with him would be very unpleasant for you. Don't take it personally *if* it happens but please consider if he is someone you want to spend your life with. 

Also, to add, the fact that your boyfriend is doing these things (letting BM control him, choosing obedience to her over your relationship because he's too spineless to fight for his parental rights) and that you are upset by them does not mean there is something wrong with you. There is something wrong with him and the situation he has created or allowed.

The problem in this situation is not your existence or your feelings. However, you are left with a choice to make. Once you've told your boyfriend how you feel, if he is unwilling (or in his case he may just be so weak he is unable) to change the situation, do you stay and try to force yourself to accept it, or force others to change their behavior, or try to make your boyfriend see your point of view? You will go crazy and your physical and mental health will suffer. Your best bet is to put all your energy into yourself until the bullsh!t of this pathetic, toxic, and broken family you've attached yourself to no longer affects you. At that point maybe you will see that there is more to life than this particular situation. That's what i'm doing and i think you should do it too.