A woman who has no relatives wants to marry. She has only one married daughter. Is it permissible that her son-in-law assumes the responsibility of her marriage as her matrimonial guardian?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If a woman has neither close nor distant relatives to assume responsibility for her marriage as her matrimonial guardians, the judge is her guardian because he acts on behalf of the governor. In a Hadeeth (narration), the Prophet, sallallaahu ‘alayhi wa sallam, said: "The marriage of a woman who marries without her guardian's permission, her marriage is void, her marriage is void, her marriage is void. If he consummates the marriage with her, she is entitled to the dowry because he had intercourse with her. If they dispute, the governor is the guardian of a woman who has no guardian." [Ahmad and Abu Daawood]

The woman's son-in-law is not a Sharee‘ah-approved matrimonial guardian unless he is a relative who is entitled to assume the responsibility of her marriage and there is no relative closer to her than he is. If the woman lives in a place where there is neither a governor nor his deputy, or she cannot go to a place where there is one, she is permitted in this case to authorize her son-in-law to assume the responsibility of her marriage, even if he is not her relative.

Allaah Knows best.