What is the ruling on helping one's brother perform Hajj with an interest-bearing loan? Will the helper be rewarded for that? And is the brother's Hajj valid?

It is not permissible to borrow from an interest-based bank to help others perform Hajj, due to the severe warning associated with it. Repentance from dealing with usury is obligatory, and it is not permissible to deposit money in an interest-based bank except out of necessity and without interest. Your brother's Hajj is valid because he obtained the money through a permissible means, and the prohibition is directed at the one who borrowed with interest, not the beneficiary.