Why has she not married despite her advanced age, and what is the way to facilitate her marriage and return to Allah?

Du'a (supplication) is the weapon of the believer, and persistence in it is an act of worship beloved by Allah. A delay in the response does not mean the du'a has been rejected. Du'a repels affliction, and a delay in marriage might be a trial, or due to magic (sihr) or the evil eye ('ayn). Ruqyah Shariah (Islamic exorcism/healing) is recommended, and one should persist in it. One must also strive for chastity by occupying time with beneficial activities, such as fasting, reciting the Quran, and keeping the company of righteous women. It is not permissible for a woman to ask for the divorce of her sister (in Islam). The husband must either retain his wife with kindness or separate from her with goodness.