Yeah it happened to me and I never saw it coming.
It got to the point that some months after I had the baby, she would come voer around 6am and DH and her would go for a walk together, then she would stay until he went to work (8:30am). INitially it wasn't too bad, it was only maybe once or twice a week and she would get DD out of the cot, feed and change her. It was nice to have a sleep in. Then it started happening all the time. On weekends she would stay even longer.
I felt that I was never getting anytime alone with DH and DD.
When I went back to work she was aksed to baby sit three days a week. By this time she was coming over every morning at 6am, the three days she was babysitting, she wouldn't leave until almost 7pm at night. Then on the weekends, she would hang around most of the day.
I wound up staying in the bedroom. I couldn't stand being around her, she would whinge about every other member of the family and when she was looking after her other grand daughter and she brought her to our place, she would bad mouth her father in front of her! (He is a dead beat, but that is beside the point!)
We struggled through many many months of a rocky marriage, I also got really sick, and having her there made it all that much worse. DH wouldn't do anything about it.
In the end I was ready to pack up my bags and leave.
MIL has this way of making the kids feel dependant on her, she won't let them do anything for themselves! I heard her go off at her 9 yr old grand daughter becuase she wanted to brush her own hair!
Now unfortunatelt my DD also seems to have picked up that same dependancy, but we are working on that.
Eventually it all came to a head when I was talking to my SIL (BIL's partner) and she was bascially venting to me about MIL and the little girl heard and ran back and told MIL (MIL would make her tell her everything that was said when she wasn't around - she would use a guilt thing on the kids). So then my hubby rang me and told me to fix it. I rang her and we had a chat. I didn't have it out with her or anything, but she has backed off a little. DH has finally realised that if he wants to be a mummy's boy he can move back home with her, I'm not putting up with that. She doesn't come over anywhere near as much and our marriage has gotton a lot better.
Nip it in the bud NOW, becuase when you have kids it will get worse.
I have a sign, that I got recently that you can hang on the door that says "Family is resting, please come back at..." and a picture of a clock with words "Another day" where the 12 should be.
You could try something like that if you don't want her coming around unannouced?
When this baby is born, and I go back to work/uni they will both go to daycare. I'm not having the same thing happen again.
I don't mind her having a relationship with my kids, but she is going to pretend to be their mother, which she would do with DD.
When we went to family gatherings, DD would have to ask her if/when she could eat something!! I didn't realise it was happening, until someone told me!
I put a stop to that straight away. DH is to blame for that, as he was happy to let her take over the parenting whenever she was around. And I still have to say things every now and then, so that she remembers that she is a grandparent, not DD"s parent.