What to expect??

So it's been a very long emotional day. Okay so my fiance had a daughter from previous relationship, the mother and him have had a rough time parenting. And on both ends there are faults. Back in October we got called regarding his daughter, her mother's bf had been arrested on meth distribution charges, and the mom ended up moving back in with the bf after he got bailed. We just went there for her bday party on the 3rd her bday was the 2nd. I couldn't or wouldn't have thought ANYTHING was going on. Today we get a call asking to please come pick up his daughter, as mom and bf were arrested and charged with purchasing materials and conspiracy to make meth, and making and distributing meth and something about something charge 2. So as soon as my fiance is off work we are going to pick her up from her grandpa's. My question is what should I expect? My aunt's been telling me be prepared for all sorts of stuff with his daughter. Gonna go get her checked out this week because we have a suspicion she was being abused. When we were there the mother was talking about how she pees herself when her hair is brushed. Which I get hurts when you don't brush for a while, I hated mine but never did I pee myself. So I'm very concerned she may have been abused. And was wondering if anybody here had a similar situation and what happened. And yes we are going to file for custody of her as well. We have our son (21 months) and our daughter (8 months) . Please no bashing. I'm just asking for advice.