If it were me, I would have some more conversations with her parents and not dismiss what they say. They probably have real serious reasons for warning you, but you need more info from them and possibly others who know Mom as well. They have known her a lifetime, and you have known her a few months. Anybody can be who they want you to see for a short period of time. I know a Mom who sounds similar in that she seems ok on the surface, but hides a very serious mental illness that does not become apparent until many months after placement. She has two children she does not parent(we do not have either of them). Just do more research so you are prepared.
FWIW, if I were your emom and found out that you were talking to my parents about me because of things they have said to you and you are looking to possibly close off the relationship because of it, I would want a different match. Trust goes both ways. Right now she is learning to trust you enough to place her child with you and you are learning to trust her to have her as part of your on going lives. I know plenty of people that have horrible messed up personal lives but maintain a very healthy open adoption. I am a mess and a crazy person half the time. Heck ask my ex how many times I have called from a blocked number and hung up. Does that mean I do it to aparents absolutely not!
I don't know the situation behind her other child's circumstances. I don't know how formal that adoption is. All things that I imagine would play a really big deal into the dynamic.
