How to I convince my younger sister to get out of an abusive relationship?

My little sister is almost 21 yrs, but she's with a guy for almost 3 yrs, and for the same time she's been crying evryday of her life. Her B.F is an entrepreneur and she ists him with his business, and has refused to get a good diploma to be financially independent. Every one has tried talking her to her senses, but she says she loves the guy too much to leave. My sister has had inferiorty complex in her teens and she obviously doesn't love herself. She has even abondoned her dreams of graduating at nursing school. My mom has caught her B.F in intimacy with other women, and my sister has even seen his B.F cheating for herself. But she refused to leave him. Her B.F's business is rumoured to be going bankrypt, and my mom caught my sister doing fraudulent papers for the , and my mom has warned her of the consequences. She claims to love the guy so much and needs to do anything necessary to help him, even if he treats her like his doormat. He has abused her verbally.