Is it permissible for a brother to marry a girl whose mother suckled his younger brother, but she (the girl) was not suckled by his mother?

"Suckling establishes the same prohibitions as lineage," as the Prophet, peace and blessings be upon him, said: "Suckling prohibits what lineage prohibits."

The suckling child alone, without his siblings, becomes a child to the mother of the milk and its owner.

If your brother suckled from the mother of that girl, and she did not suckle from your mother or from your father's milk from another mother, then this prohibits your brother from marrying her only, without prohibiting you from marrying her.

Therefore, it is permissible for you to marry her, as long as you did not suckle from her mother or from her father's milk from another mother, or share suckling with her from another woman.

It is a condition for suckling to establish prohibition that it consists of five satisfying sucklings.