When is a good time??

Not really sure where to post this but: So recently my MIL has been posting a bunch of stuff to my fb wall about babysitting when we have the baby, but the problem is, I don't want her to be left alone with them. I understand that it is her first grandchild and she is excited, but at the same time I do not trust her at all. She is not responsible, has five other children that she doesn't take care of, let a BD whose kids have a protection order against live in the same house as them, and bought drugs and did them at Christmas dinner in front of everyone, just to name a few reasons. I'm not sure when would be a good time to tell her that my SO and I don't want her babysitting, or being alone with the baby. I feel bad for confronting her but at the same time I don't want her to get her hopes up.