Is it normal if i'd like to fuck my best friend?

Hi guys!My best friend and i are friends since we were born so she is like a sister for me. Everything changed when she started to grow up so i started to fantasize about her. She has the biggest ass i've ever seen in my life. One day she asked me how big my c**k is and i was completely honest with her and she said great, and the day after that i touched her booty and she smiled and she said to try again and to grab it harder and she looked like she liked it. Since then i am constantly thinking about her and about her ass so i would appreciate if you guys will help me out with what should I do