If a family property is to be divided after the parents' passing, a mentally ill sibling is still entitled to their share. A guardian, either family-appointed or court-appointed, will manage their interests. If an amicable family settlement isn't possible, a partition suit can be filed, and the court can appoint a guardian for the mentally ill brother. It's also possible to relinquish your share to the mentally ill brother through a trust, ensuring it's managed by a guardian and cannot be sold without court permission.