What is the ruling on the increase in the number of T-shirts, which resulted from an increase in the quantity of fabric, and is it permissible for my brother to keep them, knowing that the agreement was for only thirty pieces?

So long as the contract between you was for only thirty pieces, the excess belongs to your brother. The right of the one commissioning the manufacture is only for the thirty agreed-upon pieces, according to the specifications that were contracted upon. "Istisna'" (commissioned manufacture) is the request for something not yet made, to be manufactured according to specific characteristics, using materials provided by the manufacturer, in exchange for financial compensation.