The fact that you are here in SR says that you are doing something about your problem. So this is great!
It is my belief that you need the family support, unless telling the person will lead to an abusive situation. I agree with Mariechi - he already knows or is just waiting for you to say it. You will be telling him that you are taking steps to "not be like the MIL" so this should resonate with him.
My spouse has been amazingly supportive through this - and this is a big reason why I think I have been able to get as far as I have.