I'm lucky in that department, my MIL is a lovely person. It's just sometimes that she makes that odd annoying comment implying that her way of raising her children was better. She also likes to talk about us DILs to each other behind our backs about what she disagrees about our parenting.
Example 1 - "little Tom (my nephew) was here and was very good. He didn't play up like he does with his mum" - surely a 2 year old will find much more important getting his mum's attention than that of a grandparent
Example 2- "SIL won't smack her children. Did your parents smack you as a child?"
Example 3 - "SIL won't give her 2yo the bottle. Don't you think it's great to give them the bottle and they will doze off by themselves in the cot?"