I'll Be Your Romeo, But Only If you Be My Juliet

Cliche title,
I know.

Young Alexis Snape, Severus Snape's Little sister by a year, is nothing like her big brother.
She gets sorted under a different name, which confuses her, into Slytherin but when she falls for one Sirius Black, all hell breaks loose between her and her brother, not to mention her old friends.
Her brother's threatening to disown her if she doesn't stop seeing him.
"But Alexis, our love can be forbidden! Your brother will never have to know!" ~ Sirius Orion Black.