What is your advice for dealing with a sister who rejects the guardianship of her guardians after divorcing her husband and moving to live with her mother, especially since she is threatening to become independent and live on her own? And who is her legal guardian in this case, and how should one deal with someone who rejects their guardian?

This woman should not be given free rein; rather, firmness must be shown with her. It is preferable to seek out a righteous man to marry her. If this is not feasible and she persists in her condition after being advised and warned about Allah, then she should be ostracized if ostracism benefits her, otherwise not. The authority to discipline her belongs to the شرعي (Sharia) judge to deter her, as long as she is not married.