You break up face-to-face and tell her what was wrong with her, then its another guy's problem.
If the quality of friends you keep are not worthy of dating your sister, then you have to start living a good life and flee away frm any wrong association.
I do remember one time my best friends sister was eyeing me up all day long and just acting shy.
Description: This is not always possible. No guy wants to tell his girlfriend the wrong truth.