Is it permissible for the wife to stay alone at home with the husband's brother who suffers from a mental disability and cannot manage his own affairs, in the absence of the husband?

If the husband's brother is lustful towards women, he is a mahram (unmarriageable kin) to her, and it is impermissible for her to be alone with him or to display her adornments in front of him. However, if he completely lacks desire and has no inclination towards women, then he falls under the category mentioned in the verse: "Or those male attendants having no sexual desire." Some scholars have mentioned the permissibility of being alone with him, but the more correct view is to prohibit it. If he needs assistance with food and drink, and there is no one to help him except the woman, then there is no harm upon her in doing so to the extent of the need, similar to the ruling concerning a doctor.