Help! i made out with my best friend's sister?

so the other day my friend told me to come over to play basketball. i went to his house first assuming we would walk from there, but he and a couple other of my friends already left. anyway, when i walked in and he wasnt in his room, i asked his sister (shes 15, me and my friends are all 17) where he was. she told me but she also said her parents and him wouldnt be home for a little while. i seemed like kind of a weird comment so i was like "thanks" and was on my way out when she asked if i wanted to chill in her room. so i was like whatever and one thing led to another, she was being really flirty, and we started kissing. anyway, i kind of have a crush on her (shes really cute) but i didnt want anything to go any further (it could have! she was stroking around down there and i think she wanted to give me a bj. and trust me, it was tempting to stay lol but i was afraid my friends would walk in or something). so i got up and awkwardly apologized then just went home. now i dont know what to do! mikey has been my best friend since elementary school and ive known his family forever, i dont want things to be weird now. i dont think his sister will tell anyone, but im over at his house all the time and things will just be uncomfortable now. what should i do? i dont really want to tell him, but i feel kind of gulity. and honestly we didnt go past kissing!