Well people that have adopted children, like a newborn would probably feel differently about that, its not really much different in theory.
The problem here are the adults, you can't treat the children like adults-since they would be getting the brunt of the issues between you and the inlaws. You're right, the inlaws probably don't like you. I have gathered that now, but so what you're there, the kids mother is not. Thats the attitude I took with my MIL.
BUT you should be clear in your mind..... if those kids live with you, they are YOURS, so if they/you/dh aren't going to do for all, then don't do for any. And that means the inlaws. Explain this to dh, explain it to your kids, ALL four of them. If the inlaws/grandmother brings something over, don't take it, throw it away. I did this to my MIL once and she never did it again. I said where is the such and such for Joe and Blow? She said "she just wanted to do something sweet for sally". I said well its not fair to joe and blow, so I threw it away
. That was that. I understand your dh does not want to be in the middle but he's going to have to take a stand.
IMO, you have more control in this situation that you think. Clearly, if you've been taking care of those kids, YOU are the mother. We all know that women silently rule the house. I wouldn't even worry about dh getting upset about you taking a stand against what is right and fair because you are the one that is mothering them, what you say, should go. I would lay the law down with dh and that would be it. I know its easier said than done and obviously if you've been silent all these years this is going to be hard to do. I just think you've been quiet so long about it, you thought it would get better and it didnt'.