Who is responsible for the care of the unmarried younger sister (36 years old) with a mild disability, after the death of her parents, given that there is one brother and two married sisters?

Regarding the sister's disability: If her disability is only physical and her mind is sound and she is competent, then the matter is hers concerning herself and her wealth, and she can appoint whomever she wishes. However, if her disability is mental and affects her intellect, or if she is imprudent and cannot manage her affairs well, then refer to Fatwa numbers 51328 and 45381 to determine who has guardianship over her.