Why did Nadia say yes
Because she wants her head examined!
Despite having a dream where her little dog got out of her garden and was then confronted by, and killed by, a black snake (its unblinking ophidian stare hypnotising it, no doubt . . . ) she was so in love with the idea of being with this "dominant alpha male" (the experts' description of Wanktony - not mine) that she couldn't see that she was the little dog, leaving her safe, protected home and delivering herself into the coils of Wanktony (his unblinking ophidian stare hypnotising her, no doubt . . .).
That poor girl is in for a rude awakening, but has brought everything on herself. Her sister hates him, I bet her friends hate him, her subconscious mind doesn't trust him, and I would think everyone, including the Universe, and her own brain, is telling her to be careful, and she is just sticking her fingers in her ears and singing "Lalalalala! I can't hear you."
why did Susan say no
Lifestyle is too different and she realised she'd find the isolation and being stuck in that hellhole with his mother too difficult to cope with and they'd end up miserable and at each other's throats. This way they can probably salvage a friendship, possibly "with benefits" as I believe the saying goes.