MIL undermines parenting style

So before I had my baby I noticed my mil undermining her other daughter in laws parenting style and routine and rules. Making herself look like the spoiling grandma and my sister in law like the mean parent. I felt bad for my sister in law. So now that I’m a mom I’m sure she’ll do that to me. I mean I’m making innocent comments like “the baby is looking chunky “ and she’s like don’t body shame her. Putting words in my mouth., yet she said my baby feels solid and heavy?!? Ok lol so how do any of you deal with mil or anyone who undermines your parenting style ?