I almost had sex with my friend's brother?

i was at my friends house for a sleepover. it was the morning, when i needed to get change. her mom told me to go to her brothers room (he is 15 and i am 13). i always had the feeling that he didnt like me considering that he never talked to me and always made a face when i was around. i was about get change when i saw him. i said sorry but he said it was okay he was without a shirt which got me turned on. we started flirting when he asked to see my vagina. i said ok and took off my pants he then told me to sit on his lap so did. he asked me to spread my legs. i did that and he started feeling me i started moaning, so he took off his shirt and pants and i took off my bra, he sucked on my tit and we were completely naked on his bed. his mom knocked on the door and asked if i was ok. i said yes and he climbed ou the window. the other day i asked him about that and he said to keep it our little secret. what should i do should i tell my friend?