A valid question was asked, are you talking about a male-male
relationship?
Male-male, you either end up as bitter enemies or one of you stays
in love while the other moves on.
Male-female, either the guy falls in love and the girl moves on or
the girl gets demanding and the boy feels trapped and runs.
Female-female, one will decide to try men and the other will end up
waiting.
Edited: I think this advice may be too late for you. But if you are
close friends, do something to signal a change of your intentions.
Start buying flowers, refer to her as your girlfriend. Give her
some time to adjust her mindset to being your girlfriend. Then
she'll either stop you or let you continue in your courtship.
Otherwise she'll feel like she was molested by her brother.