Is a marriage valid if it takes place with the permission of the girl's mother's husband, and with the knowledge and approval of her brother, in the absence of a father or grandfather for the girl?

The stepfather is not the legal guardian (wali shar'i) of a woman. Rather, her marriage is to be solemnized by her legal guardian, who is her closest male agnate (asabah), then the next closest. If a woman is married by someone other than her legal guardian, the situation is one of two: First, if it is done with a proxy (tawkeel) from the legal guardian, then the contract is valid. Second, if it is done without a proxy from the legal guardian, which is known as "nikah al-fudooli" (marriage by an unauthorized person). Scholars have differed regarding its ruling: some consider it invalid unless ratified, while others deem it suspended pending ratification. In this case, if the woman's brother authorized her stepfather to conduct the marriage, then it is valid. If he did not authorize him, then the matter follows the scholarly اختلاف (disagreement). However, if her guardian is aware, approves, and ratifies it, then the contract is valid.