My MIL has no life outside of my kids. (My SIL isn't much better.) My MIL has done crazy things!
She would come to just hang out at our house when the kids were babies. Never offered to help with anything. I used to take the baby into the bedroom to nurse and "fall asleep" with the baby just so I could escape her in my own home.
She showed up at the portrait studio to watch us get 6 month photos taken of our daughter.
When my husband is going to pick the kids up from school he first has to leave work on the SW side of town, drive to my MIL's house on the NW side of town to get her, then drive to school on the NE side of town. Most times after they get the kids they drive her back home and then come home so she only sees them in the car for about 15 minutes.
If my husband takes the kids to school they first stop to pick up my MIL. I'm always afraid I'm going to get a call about the lady stalking my kids on the playground because she just hangs out on the edge of the playground to watch them play.
When we left for a vacation last month she showed up at our house at 6:30 am so she could tell the kids goodbye.
I have "forgotten" to mention things that the kids are doing so that we don't have to involve her. She has accused me of keeping the kids away from her. (Not so much keeping the kids away from her, but giving myself breathing room!) She has berated my husband because we haven't included her in things. She has bad mouthed me many times. She and my SIL speak another language and when my husband is out of the room I swear they are talking about me. She has made a point of telling others that she doesn't get to babysit for us very often, yet at least twice she refused to or backed out of babysitting for us because my husband asked her to do it instead of me asking. It drives me crazy! She can be very passive aggressive.
I wish I could say it has gotten better, but it really hasn't. She rarely comes to our house if I'm home. She waits until I'm gone and then comes to hang out. She leaves almost the minute I walk back into the house. I have been able to set some boundaries, but I've also had to learn to keep my mouth shut and just put up with things sometimes. My husband gets a lot of dirty looks from me!
Stand up for yourself, but also know that sometimes you have to compromise to keep the peace. Decide on what are non-negotiables for you and what are things that you can give in a little. For example, everyone has to be gone by bedtime. Saturday's (for the most part) are just for the four of us. One of our vacations each year is just for the four of us. When my parents come to visit, they are the priority because they live 1000 miles away. I have to make a lot of compromises that I'm not always happy with, but it keeps the peace. Good luck!