My best friend when we were teenagers spent about a year with her step uncle. He was the step brother of her mum. He was also about 10 years older than her. She was 15, he was mid 20's. Looking back I do wonder why this was something the adults around us didn't have concerns about. It wound up being quite a toxic break up. He's not the WORST choice she ever made but he was the first truly wrong choice - which makes me wonder how much of her later tough times could have been avoided if any of the adults in that situation could have stepped in.

Also knew a girl who was fostered by the same family from about 10 years old - she started dating her foster brother when they were teenagers. I don't understand how the adults in that situation let that happen. She was still living with them in the same house. They were still foster siblings. I'm not sure it ended well either. She did end up leaving that foster family and moving back in with her mum.