It's Always Been Lily

Briony Potter is the twin sister of the famous Harry Potter, except they are very much different since she's a Slytherin and he's a Gryffindor. She has red hair and blue eyes. Briony has always wished that her and her Harry could be close and have a relationship like they did before coming to Hogwarts, but since they are in rival houses Harry has made it his mission the past four years at school to pretend like they aren't even related.
Briony is dating Draco Malfoy, and eventually finds herself in a very complicated situation where she ends up having a secret relationship with a certain professor; Snape.
Briony has to juggle her schoolwork, brother issues, and love life all at once, which leads to much heartbreak and problems.
Can Briony survive her fifth year at Hogwarts or will this be the one to take her down for good?