This mistress had talked to her slave about consistency and she felt that she had told him enough. He needed to have gotten the message by now but he did not and she was disappointed in him. That is why she was interested in making sure that he would not forget the message she had for him. He had to eat her shit and drink her pee for that to happen and it did.
This mistress needed to torture her brother in law because he had tried to mess with her. The mistress did not want to let the guy get away with trying to hit on her. She felt it was too fucked up for him to do so and she felt disrespected. So she punished the guy by cruelly shitting on him. That is how he learned never to do that again.
Mistress Nemezis does not like inconsistent people and she did not like the fact that this guy was one. He was trying to forge a relationship with her but she did not want anything to do with an inconsistent guy. That is why she opted to whip him and cruelly dominate him. He was whipped while he was naked and that is how he learned his lesson the hard way.