Is a woman required to wear her full clothing in front of her husband's brother who suffers from a mental disability, and what are the Islamic rulings and limits for dealing with the extended family?

A woman must cover her entire body from non-mahram (definition) men, and this includes the face and hands. The husband's brothers and his relatives — other than his fathers and sons — are considered non-mahram to her, and she must cover herself in front of them. This is because the Prophet, peace and blessings be upon him, said: "The in-law is death," meaning that the fear and temptation from him are greater than from others due to the ease with which he can approach the woman and be alone with her.

If the husband's brother suffers from a mental disability and does not comprehend matters concerning women, nor is he inclined towards them, and has no sexual desire, then he falls under the category of "those male attendants having no sexual desire [for women]" [Quran 24:31], and the woman may uncover in front of him what she uncovers for her mahrams. However, if he is aware of women's matters and is inclined towards them, then he is like any non-mahram man, and she is required to observe hijab (definition) in front of him.