What is the ruling on looking at the anus of one's sister with compulsive lust, while being unable to restrain oneself?

It is not permissible to look at a forbidden woman (mahram) with lust, even if she is covered. Nor is it permissible to look excessively at a forbidden woman without necessity. It is stated in "Prohibitions of the Tongue, Hearing, and Sight": "And a woman's looking with lust from above clothes is among the obligations to be avoided. It is not permissible to repeatedly look at a young forbidden woman unless there is harm."