Is performing Umrah in the month of Ramadan sufficient as a substitute for performing the obligatory Hajj on behalf of a deceased person who made a will for Hajj to be performed on their behalf?

The virtue of 'Umrah (definition) in Ramadan (definition) is that its reward is equivalent to the reward of Hajj, but it does not suffice for obligatory or voluntary Hajj, nor does it waive it, by consensus. Scholars have stated that this equivalence is only in reward, not in sufficiency. So, whoever has Hajj due upon them, 'Umrah in Ramadan does not waive it. Therefore, it is not permissible for your brother to replace the Hajj that your mother willed with an 'Umrah in Ramadan. If he does so, it will not suffice, and he will bear the sin of altering the will, and he will be liable for the money, and your mother's money will remain a debt upon him. If your younger brother refuses to perform Hajj with that money, then appoint another trustworthy person to perform Hajj with it.