Its really hard to post about situations like this on forums because there are so many details that get left out to give people the real feel and whole picture of the situation.
I could not even begin to go into detail about the terrible situation my husband and I have suffered through with his mother who was widowed 20 years ago. We have no alies whatsoever with his three other siblings(all in their 30's) as they all live together and must think the same. Being a widow does not mean you have a right to be cruel to people and have it over- looked.
My husband and I started family therapy together before getting married to make sure we stayed on the same page. She would never attend therapy with us. Most people who have little insight or emotional intelligence are not very open to therapy. If your MIL had any of this it is likely she wouldn't treat you this way to begin with.
The best you can do is never stoop to their level, hold your head up and be polite. This doesn't mean not standing up for yourself or being blatantly disrespected-just maintaining the good self you are despite how you are treated. Its so worth it for both yourself and you are giving them absolutely no amunition and you can have a clear consious.
We have a 2 year old now and I understand how uncomfortable it is to share your child with people who are cruel to you. It feels emotionally un-safe. We visit her with him, but do not leave him alone with her (for other reasons too difficult to get into.)
I understand the rage you feel and that you are just venting here. It takes a lot of work, but its not too late for you to get rid of the anger. Its hard to forgive when someone doesn't say they're sorry, but try to feel better so the anger doesn't consume you. I know its hard. You really have to be in someones shoes completely to undestand their situation
Sorry for the long post. Stay focused on the good people you have in your life.