What is the ruling on uncovering the face, looking, and sitting in the presence of the wife's mentally disabled nephew, who is unable to discern, knowing that he is 17 years old, and what is the ruling on past and future actions?

There is no harm in looking at the face of a disabled adult if it is free of lust. However, uncovering the face in front of him is not permissible, because his disability does not negate the obligation of veiling from him. His ruling is like that of an adult insane person, from whom one must veil. The ruling for an adolescent is like that of an adult in many matters, and some scholars have explicitly stated the obligation of veiling from one who has reached the age of ten. As for what has passed regarding uncovering the face or shaking hands due to ignorance, there is no sin in it, for error and forgetfulness are lifted from the Ummah.