BFSS 

(n) Best friends sister syndrome - a diease that results from dating your best friends sister. Symptoms- your best friends sister seems hotter than usual and you have the urge to ask her out. Warning may result in death in some rare cases

Dude, your totally going to get BFSS from dating his sister.

BFSS by Gil Jacobson January 14, 2010

bfsspl 

best friend sister soulmate platonic lover
created by cat n kenz.

don’t mess with my bfsspl!

bfsspl by misseverywhere November 4, 2023