16 isn’t child really. I’m 16 and know thousands of 15-17 year olds and no one would think this is ok, she took a bad decision in her teenage and even after becoming adult she decided to stay with it? Its both her and her groomer’s fault. She was a victim back then but now she can take action but she won’t? Its ok its her own life let her live how she wants.
About the property thing i think its up to your parents to decide who will get what. If she doesn’t talk to your parents for years and suddenly after their death she comes to claim property, its so shameless. If you all get on good terms and relationship then it might be ok for her to get if your parents wants.
And we are hearing this situation from your pov. Maybe from her pov the situation looks different? Who knows, for now this is what i think.