This hurts me a lot because i been hiding how i feel for years about this girl?

She is my bestfriends sister I like her a lot but she is younger than. I been hiding this since i was little but now i grew more feelings for her. I know that i can't like my bestfriends sister but it feels as a dagger going into my heart because i havent told anyone how i actually feel for her. I dont want to ruin my friendship because it means more i dont like going over there because of her because i think more about her and question myself. I know she likes me too but i cant were both holding the likeness back as i can tell. I never intented on telling her how i feel and i dont think she will either but we both know by the passion we give each other. I know my bestfriend and her father would never approve of this. What should i do?