Choose best friend or my y love?

I am thirteen and today I went to third base with my bestfriend(13)'s brother (16) we rnt actually boyfriend and girlfriend but we r always out together and he is always@my house. My bestfriend is not aware of this relationship and I am not sure what to do- I really really really love him and her(as a friend hah)- what do I do? She'll be so pissed off if she finds out!but I can't do this any longer!