I need guidance on an issue with my 17 year old adopted son

In 2018 my daughter's father passed away. My daughter has a younger half brother from her father and former wife after mine and his relationship ended. My family ended up moving into the same neighborhood as my daughters ex step mother and half brother in 2016. Shortly after we moved in her half brother who then was close to 12 started coming to my house more and more everyday till he was coming and going to school from my house. His Birth mom was addicted to drugs and was known as the meth house of the neighborhood. Shortly after his Dad succumbed to his illness and passed away. We moved and took the kid with me. His mom signed temporary guardianship over to me and went about her life and has reached out to me maybe twice since then. I call him my son because he is part of my life and family. So I will refer to him as my son from this point forward. He is 17 now and a senior in high school. He has a slight learning disability and a moderate behavior problem. Which I deal with but what I can't deal with or better yet what I don't know how to deal with is this; about a year and a half ago my now 19 year old daughter who lives athome with me which is not his sister came to me and said she thought he was being weird and she thought he was trying to touch her while she was asleep. She said she would wake up and he would be around her sleep area and would say he was looking for something. I honestly thought she might of been imagining it and she wasn't 100% sure it wasn't a dream. I never ever would think he was capable of something like that. Shortly after that my son and 19 year old daughter stayed the night at their sister's house and she called me hysterical crying telling me she caught him doing it again. So I picked her up and she told me what happened. My son is very good at acting like he has no clue what she is talking about and since she has some mental issues herself it was unclear if she wasnt making things up in her head. I wasn't sure how to go about this issue or if was a her or him issue to deal with. Then one night I was asleep in my bed and I needed to use the bathroom so I jump up out of bed and almost stepped on my son who was laying directly beside my bed in the floor and I didn't think anything of it at first I thought he was sneaking my cigarettes since he was caught smoking around that time. After that I was sick and in the bed and woke up to someone touching the small of my back and when I sat up my son was on a pilet in my floor watching TV. Once again I thought I was dreaming. Then it happened again and I woke up and yelled for him to get out of my room. And started sleeping with my door shut. And my daughter would also. I told him he was not allowed to go in anyone's room if they were asleep. I never directly brought up the issue because I don't know how to. Well the incidents stopped for awhile and I thought he was grown out of whatever phase he was going thru till recently I was sleeping on the couch and felt that familiar gut wrenching feeling of someone touching the small of my back and I jumped up and he was there and I told him to get away from me doing that. I don't know what to do he's almost 18 now and his attitude is changing for the worse. Everytime this has happened that my daughter and I are aware of it's the same thing everytime. I don't know what to do and whether or not I'm exposing my already mentally ill daughter to something potentially dangerous or what do I do? I'm so lost.