My sister-in-law is like this, and got worse with each pregnancy by the stories I've heard. A week after she had her last baby a high school friend was killed by a drunk driver and between that and postpartum depression she's been incredibly mellow and nice. She actually apologized for being rude and catty to me ever since I've know her and said she needed to get her priorities straight when she moved back in with her mom. I've tried to be very neutral except for that one comment, but for the past three years it hasn't helped a bit. So I mostly ignored her and dealt with her kids instead, whom she told I hated them.
I'm sure its got to be hard for a first time mom to worry about their mother-in-law's opinion especially if you two didn't get along before or weren't especially close. As a new mom you don't have time to cleanup the house or yourself and that probably makes her feel like a failure. I know I anticipate a problem with my mother-in-law if I ever have kids. She's used to being able to practically raise her other three grandkids, and she wasn't a spectacular mom to start with and I won't have that. We get along so long as she doesn't try to force herself and her ideals onto me and act like nothing my husband says is right, but if she did that about my kids I'd move across the country and leave no forwarding address. She has no boundaries and thinks she knows everything better than the next person. Add hormones on top of it and I'd definitely look like a raging female, lol.
