Sleeping w My Best Friend's Brother

I recently got kicked out of my house (my dad got tired of me and kicked me out of the house), so I reached out to my best friend and asked her if I can crash at her place for a few days. My bestfriend is actually two years younger than me but her brother is the same age as me. I was actually his friend first, but I ended up getting closer to his sister. After a week my friends brother started to text me (even if we were in the same room or two rooms away) and flirt with me, since we were always flirting I never took it seriously. Until one day, my best friend was out at work, she had the night shift. So her brother and i hung out. One thing led to another and we slept together. Before my best friend was back I ran back to my room and I felt so guilty. Weeks have passed and I still haven't told my best friend. I moved out with an aunt, and her brother keeps texting me. And wants to know if I was down to have sex again. I am totally down, but I'm not sure if I should do it again. What should I do???