Sister in toxic relationship, how do I help?

Hello community,

I recently found out that my sister is in a bad relationship with a man who isn’t even her boyfriend. My mom and I have tried speaking with her but she denies and down plays everything. We don’t know how to help her and it’s clear that she’s been brainwashed. If anyone has advised on how to help please share with me . We love her so much and just want her to come back home.