Gracey,
I don't post on many threads in this forum, but I do find myself drawn to reading your posts. I wanted to comment today because I really see so much in your posts that show how strong you are, and what a good mother you are to 3 children - not to only two, or only one. The H and MIL are not doing your little girl any favors by treating her as though she is better or different from her siblings - eventually, she will be an adult and facing the 'real' world in situations where daddy and grammy cannot make things perfect for her.
Keep doing what it is that you are doing. I agree with everyone else here, you are just amazing - I can't understand why H and MIL can't see that, except that they are just not right in the head. My MIL is far, far from my favorite person, but she treats my son just as well as her biological grandchildren.
And I agree about divorce, it is not an ending, it is a new beginning for you - and if that's what you want, you deserve it. You are a wonderful woman who deserves the best that life has to offer.