I have the same problem with mine.  when I married my husband he said that he  barely talked to his mother. She was not there when we got married and did not even see our son until he was almost a year old.  This being my second marriage.  My first husband was the only boy and had only one sister.  His mother was not rude or really cared about our daughter, she only cared that her son jumped when she said to.  My second mother in law is everything yours is rude even to the point of telling me to put one of my kids on a diet. (oh ,my second husband is a only child) My advice also is that if she is there leave.  If you don't feel comfortable leaving your son with her and your husband, then take him with you.  I also feel that it is your husbands job to talk to her about the situation or both of you talk to her together.  Make sure if you and your husband talk to her together that it is a united front and you have really thought about what you want to say. 

I too share your pain, but when my son was born she would not let me be his mum. it was horrible as i had really bad post natal depression and was struggling to breast feed him.

Mother in laws are hard to deal with in most of the time as all they want is the best for their kids and nothing is.

I would try and not see them and if she asks why say you see all of us. if its the first baby give a bit as it is a new thing and they do have to find their place as well. or the other is if you have a sister get her to babysit (any one really) then go over. that will screw her up. lol

I hope it works out for ya.

Yes, severely limit your contact with them.  They weren't interested in having a relationship with your family (you and your husband) prior to the child.  There's no reason why they have to see you all of the time now IF IT IS CAUSING YOU UNHAPPINESS.  Parenting is hard, especially in the early days and you need support right now.  You do not need to be undermined.  Make decisions at the moment based on YOUR needs and that of your husband and baby.  Sure, don't cut them off completely but limit the contact to something that you are comfortable.  This is your family.  This is your choice.  This is your sanity.