Stressed about the talk

My husband and I adopted his little sisters two daughters (1yrs & 2.5yrs old) because they were in foster care, going from to house. They had been in foster care for almost two years. They had been in 3/4 different homes. The last home, the placement parents were abusive to the older one, the younger one was their family but the older one wasn’t. My husband and I had been talking about trying to find a way to get the girls so we could foster them hoping the bio mom would accept help and get clean to get her girls back but she kept relapsing. One night about 9ish the case manager called us, being the last relatives available, she asked if we’d take them in emergency. The placement parents told her they didn’t want them anymore and to come get them. Of course we accepted. That next morning I picked them up from cps and the older one was super skinny and bruised everywhere, they stated she was clumsy and would fall a lot. After having them a couple weeks, she was not clumsy at all. She was so hungry as if she wasn’t being fed and she would tell me horrifying stories about the placement mom being abusive to her. After 6 months of fostering them the case manager said we had the option to adopt because bio mom wasn’t doing anything anymore was going to terminate her parental rights. We accepted of course. The next month they started an emergency adoption and we adopted them. I’ve had my daughters for a little over 2 years now. Bio mom being family would still come around but was too obviously high all the time and started trying to sneak them alone somewhere and would joke about kidnapping them and going to Mexico. So we started watching her very closely. Meanwhile, the older one decided to forget everything and is now a very happy normal child, finally. But bio mom keeps trying to tell them who she is and making them confused so we tell her to not say anything to them until they’re older and ready to ask questions, to just let them be kids for now. She refuses and keeps trying so we finally told her if she wanted to stay in their lives that she had to get help in rehab or choose to lose them for good. She chose not to get help and ran off to Vegas. So we assumed she made her decision. It’s been almost a year now since we’ve seen or heard from her, our choice, and now she’s stalking me on Facebook and making fake accounts to say terrible things about me. So we decided to keep away from everyone involved with her, family or friends. But now I stress about when they do start asking questions and want to see her, how will we handle it or what would we say? I don’t want to keep them from knowing her but she’s poison. It’s their right to know her and see her and meet with her but being so estranged with her now stresses me out to think of how to handle her when that time comes. My husband doesn’t even want to think about it yet. But my brain won’t stop stressing about it. What is your advice?