How do i tell my best friend's sister i love her?

What is the stumbling block? You're attracted to your sister's friend, not your sister. People date friends of siblings all the time. Sometimes, they even marry them. Is there a serious age-difference problem here? If your sister's friend is inappropriately young, then tell no one and wait for her to grow up. If you are having inappropriate sexual thoughts often, you may wish to seek confidential counseling, especially if you think you will not be able to control yourself and may act upon those feelings.