Do I tell her sister?

This is how it is: I have a crash on my friend's younger sister. She is about 20, i am 22. The thing is I am really good friends with the older sister (my platonic friend). So do i tell the older sister, who is a really good friend of mine that I have a crush on her baby sister? We are generally good friends, but I just don't know if telling her this will soil our friendship...but I really want to tell her! I haven't started dating her younger sister, I just like her from random meetings!!