Is it permissible for the brother to seek guardianship over his mother and sister, who have special needs, especially after the mother appointed the sister's maternal uncle to manage her affairs and properties, and disregarded the brother's share of the inheritance?

If the mother is sane and mature, she has full disposal over her wealth, and she is not obliged to bequeath anything to her children during her lifetime, unless they are in need, in which case their maintenance becomes obligatory. It is permissible to favor some children in giving due to need, to the extent of the need. Anything beyond that belongs to the legal heirs.

As for a daughter with special needs, if she is sane, no one has guardianship over her wealth. If she is insane or improvident, her father is the guardian of her wealth, then the father's executor, otherwise the judge is the one who appoints a guardian for her.

The guardian may only dispose of the wealth in matters that are necessary. As for donations, he may not dispose of anything in that regard. The fact that the questioner is the man of the house does not mean he has guardianship over the wealth; rather, guardianship belongs to the father, his executor, or the judge.

The mother or sister are not obliged to inform the questioner about their wealth or properties, unless the questioner is appointed guardian over the sister's wealth by the judge.