What is the ruling on how to act in a situation where an uncle is molesting his niece, given that the uncle is seen by people as "pious and devout," which makes reporting him useless?

This grave transgression on the part of the uncle necessitates advising him and reminding him of Allah Almighty, and clarifying the dire consequences of these actions. If he persists in his error, then the daughters must be wary of him and treat him as a stranger. It is permissible to shun him due to his wickedness, and this is not considered a severing of kinship ties.