just cause I quickly misread loquere's thread title
Bestfriend's Older Sister...
My title might be a young lass suddenly discovering her bisexual side (seeing her bf's sis nude sunbathing? sharing a room on a trip? in a hot tub? accidentally seeing the sister fucking or sucking the sister's man, or better yet, a woman?)
A more twisted version would have the protagonist a gay male, discovering HIS bi side when he is shockingly hard at the thought or sight of the sister...