For whom is Hajj more befitting: for the deceased brother, in fulfillment of his previous intention, or for the deceased mother, who did not perform the obligatory Hajj, or is it permissible to combine them both in the intention?

First, performing Hajj on behalf of the mother is for her righteousness and because Hajj is obligatory upon her, even if her Hajj might have been performed before puberty. It is permissible to change the intention of Hajj from the brother to the mother. Hajj can be performed for the brother in the future, or someone can be hired to perform Hajj on his behalf. It is a condition for whoever performs Hajj on behalf of another that they must have already performed Hajj for themselves.