Is it permissible for the brother of a woman divorced by her mother, or her seven-year-old son, to be a mahram for her in marriage?

The maternal brother is a mahram (a non-marriageable relative), but he is not a wali (guardian) for marriage according to the majority of scholars. The guardians for a woman, in order, are: her father, then her paternal grandfather, then her son, then her full-brother, then her half-brother (from the father's side), then their sons, then the paternal uncles. There is no guardianship for relatives who are not asabat (agnates), such as the maternal brother. A minor child's guardianship is not valid. If a woman has no guardian to marry her off, then the one who marries her off is the ruler or his representative.