general rule of thumb for friendzones
if you really are in the friendzone, would you still be friends with this girl if there was no possible way to become her girlfriend?
if yes, keep being her friend and date her sister if you friend isn't going to
if no stop being friends with her because either you're a stuffty friend who's in it to get your richard wet or she's using you, and then date her sister if your friend isn't gonna make a move