Have you tried meditation?
You cannot control your MIL's behavior, but you can learn how to set boundaries regarding her behavior and how to work on your own anger.
My grandmother also often has those negative, smarky comments. Sometimes, I'm not sure how my aunt deals with it. LOL. I know sometimes it makes her pretty angry. I am more used to dealing with it, so it doesn't bother me as much. Sometimes, she does make me very angry. I then have to limit my contact with her, or tell her to cut the comments. Other times, I'm able to realize that it is her nature. I can sometimes laugh it off and not take it personally.
A lot of it depends on how I'm doing otherwise. Or, it might have to do with how much I've been talking to her/seeing her.
I don't have a lot of assistance with this because I have a different reaction to people who are like that. I'm more likely to do anything for the name of people getting along, so I'm bad at standing up for myself. Rather than showing anger outwardly, I turn it inwardly. Then, I have trouble like depression/anxiety, etc.
I do know that a good step is to work on standing up for yourself in healthy ways. You can have boundaries as to what other people can do to you, and then ways to assert those boundaries. Of course, this is either said than done.
Also, maybe you just need a break from stress and pressure. I can imagine it would be stressful to have a 9 month old in the house. What can you do for yourself so that you feel better?