It's only "laughably bad" if you've never known someone like that and think it's unrealistic. Trust me, there are girls out there who will "hop on literally the 1st dick that presents itself" and they don't give a flying fuck what anybody thinks about them for it. I also think there's an element of her doing to try and seduce her brother because, being in college, she wants to do a lot of sexual experimentation and incest is clearly high on that list of priorities, so by having sex in the same room as him she was hoping to get his attention.
Also, to be able to say "no" to her would mean the MC doesn't get pushed into doing things outside his comfort zone which will ultimately help him in the long run and he also doesn't get important, practical sexual advice about how to please a woman which will also help him. Therefore, for the purposes of the MC's character development, Nikki must be an integral, unavoidable part of that.