Who inherits the mentally disabled brother if he has a married brother with no children, a married sister with children, a living mother, and paternal aunts?

Inheritance is established upon the fulfillment of its causes and conditions, and the absence of its impediments, whether the deceased or the heir is sane or insane, young or old. So, if the mentally disabled brother dies, and his heirs are his mother, his brother, and his sister, then the mother receives one-sixth due to the presence of multiple siblings. The brother and sister then divide the remaining wealth by way of ta'sib (residuary inheritance), with the male receiving the share of two females. Aunts or the sister's children have no share. It must be noted that matters of inheritance are serious and intricate, and it is not permissible to rely solely on a fatwa. Rather, they must be referred to the Sharia courts for investigation and verification of the existence of other heirs, wills, debts, or other rights that take precedence over the heirs' rights.