If the two are blood related I don't think this well become ntr. I hate ntr, but I think he might see her getting fucked by the dude and later confront her about it. She might feel bad and explain that she can't have sex with him anymore because she has a boyfriend and the relationship they share is wrong and forbidden. The main MC might change her mind with sex of course and make her break up with her boyfriend or stop seeing that guy that showed up at the end. But, will have to see in the next exciting episode of DBZ, lol.
Base +6, Great Bunga -4, Geosage -10, DonPain -6, comradierie -2