Is committing adultery with the daughter of one's wife's sister or brother considered like committing adultery with a mahram or with a non-mahram?

The wife's niece and her sister are not considered unmarriageable kin (mahram). Adultery, in general, is one of the gravest sins that incurs Allah's wrath. Its sinfulness is even more severe if committed by a married person. If it occurs with the wife's relatives, such as her niece or sister, this indicates baseness and depravity.