What should I do about the person I kissed?

Okay well I recently went over a close family friends house and they have 4 adopted kids and I am pretty close with the girl and the oldest boy. I have atayed over only once because I just recently started talking to them. The night I stayed there the boy(josh) sent me a text when we were watching a movie downstairs with his sister who is a year older than me. The text said you should share my blanket with me and put your hand down my pants(: now we have texted a lot before and we did sometimes talk like this but I didnt think it would lead to anything. When I got tthat text I got really nervous but I also wanted to. I don’t know what got into me but I felt something I never have before and I really liked it and no I dont mean I was horny. I did what he asked and later he asked me to go upstairs with him and not thinking I went we toled his sister we were just going to get food and when we were up there he kissed me! It was amazzzing. We went downstairs and returned to doing what we were doing before. Later on he sent me a text and asked me if I would blow him and I said yes. When his sister fell asleep we went upstairs and I did. We kissed and returned downstairs. After he went to bed I couldnt helo but replay the events in my head and I still do even though it happened likt 3 weeks ago. I know sounds like a fun night right? It was but my problem is he has a girlfriend and I will admit I did know this before I did all that stuff but I just I don’t know I guess I didnt really care because its not like I was cheating. But now I feel bad because I know he doesnt like me like that and I dont want to be his girl on the side. We couldnt go out antway because the family is so cloae to mine and it would be weird for our families. But I liked doing all that stuff with him I just feel like I shouldnt have im going over next weekend and im sure he will ask for it again and I want to say yes but then again I dont. Please help me and leave your advice!! Thanks