Is it permissible to allocate additional shares of the house to a brother—beyond his share of inheritance—in exchange for his financial contributions to the father's medical treatment and their travels, given that some heirs agree while others refuse?

There is no blame upon you both for agreeing to allocate shares to your brother in the house; in compensation for the expenses he incurred for your father, and in recognition of his kindness. Your sister's husband's disapproval is irrelevant. A father may allocate a gift to one of his children if there is a valid reason, such as that child's special need or infirmity, or his preoccupation with knowledge, or the immorality of another child, or if the rest of the children agree, as Abu Bakr and Umar did with some of their children. However, the transfer of ownership and possession must occur during the father's lifetime, so that it does not become a bequest to an heir, which is not permissible unless ratified by the rest of the heirs.