What is the role of the guardian of his brother with special needs, and should the brother's share of the inheritance be deposited in his name or in the name of the guardian, in the presence of a mother and siblings?

A brother has no guardianship over his siblings' money whatsoever. That authority belongs solely to the father or his appointed guardian. If the father dies without appointing a guardian, then the guardianship falls to the judge.

Guardians and custodians of orphans must manage their money in a way that serves the orphans' best interests. If they invest it, this must be done in accordance with Islamic Sharia, by paying zakat and avoiding usury. It is recommended to invest the money so that zakat does not deplete it.

Furthermore, the money should be registered in the owner's name to avoid discord.