Can I get custody of my disabled adult daughter?

My 24 year old daughter was disabled as a result of a car crash when she was 11. Her mother and I were married at the time and cared for her jointly until we divorced when she was 18. Since that time, she’s been living primarily with her mother and I have had weekend visits with her in my home. She will never be able to care for herself. Her mother and I never pursued any type of formal guardianship because there didn’t seem to be a need to do so. Her mother recently remarried and the new husband is not kind at all to my daughter. She tells me how unhappy she is at her mom’s home and that she is scared of him. She wants to come live with me but her mother will not consent. What do I need to do to get custody, or should I be pursuing some sort of adult guardianship?