What should I do if I want custody of my niece?

I am inquiring about my partner, who is my niece. She has a uterine prolapse problem and also wants to continue her studies. She is currently studying in the first year of MCA, but her mother doesn't want her to study. Her father is completely indifferent and unhelpful in this matter. I, as her uncle, have taken responsibility for her. However, her mother has accused me of being in a relationship with her. My problem is that I want to live with my niece and take custody of her, but I am unsure how to proceed. One part of me says I should get married, but legally, both of us are unmarried, and this would not be appropriate.