When my daughter was 1, on the heels of a 2 months journey thru the NICU, several surgeries and then a good span of time for us to get on stable footing of "normal" life with a baby, I suddenly felt enormous fatigue. Had to nap during the day, general low energy. My husband's sister and boyfriend were in town for the holidays. While I was upstairs resting, DH told me that she was updating the family that I'm not adjusting to motherhood well, that it's a bit too much for me to handle (not cut out for it etc...) Little did everyone (save my husband) know, I was pregnant with #2.
Any opportunity to cast judgement and broadcast my not measuring up, she'll be on it like a mosquito to dung.
When number 2 was born she would try to grab (snatch!) her out of my hold without asking or any kind of interaction with me. I knew what to expect however, bc she did this with #1 so I had my responses all premeditated. I also had to be firm when she wouldn't give her back to me (crying in her arms and 20 mins past her bed time). "MIL. Give. Me. The baby...."
The thing I've learned from so many years of being on the receiving end of pettiness, is that in time, you know what to expect, the behavior is also perceivable to others, and it ceases to bother me too much. So long as I remain honest and kind, she looks conspicuously petty. And on quiet days from a healthy distance away, I'm deeply sad for her.