D.,
You have gotten some wonderful advice, and I agree, do your best to be nice and include your mother-in-law. AND, DON'T TAKE IT PERSONALLY. Chances are it didn't matter who her son married......she just can't stand sharing him. This is about her insecurities, not you. Keep that in mind and it will help you be kind.
If she is good with your children (when you have them) then let her in. Be as nice as possible, but unlike the others I don't think you have to ALWAYS take the high road or be a door mat. If she ever says anything unkind to you I would suggest you very gently and kindly talk to her about it. Take every opportunity to show her you are not a threat and want her to have a good relationship with her son. That you want to be part of the family, not steal her boy away.
I think that if you are patient and kind you will see a change in your mother-in-law. Honor her, consider her, include her. Women do that for other women and you will forge a relationship, but it could take awhile, so please be patient.
and tell your husband he doesn't have to share anything negative his mother says to him about you. That will not help anything..........and also ask him not to share everything you say with his mother!