Is it normal to fuck your sisters best friend

My sisters friend is really attractive and hot. at first when i met her she was shy and she barely spoke when i was around. a few days later i spoke to her again and told me if i was into her because i kept trying to talk to her so i told her i actually was. my sister had warned me about asking her out. i tried for a few days but i couldnt avoid her because she came to my house everyday after school, and suddenly i noticed that she was giving me signs about her being into me. but it wasnt all of it because i caught her in my room one day and she placed an envelope under my pillow and she left. so i opened it and saw pictures of her nude. i was like what the hell but i said in my mind i would bang her for sure. my sister told me she caught her coming out of my room and told my parents that me and her had something on. so they stopped letting her come into the house, so when i went to school i found her and she told me to follow her and she took me to her house and once we were in she started kissing me and stripping me. i was like what the heck and we went to her room and started fucking real hard. so now we have been doing it for a few weeks 3 times a week. i started feeling guilty because she is my sisters best friend and i would want to ask if i should tell my sister or keep banging her best friend? shes is blonde, nice tits, great ass and pussy, 22 years old, im 23. nice body.