I'm still sticking with the theory that if they indeed won't end up together, and this hasn't just been a big fake-out by Sumodeine, it's because something bad is going to happen, whether that's Taylor dying, or Nikki dying, or Nikki killing Ashley and going to jail, basically anything that is going to cause them to be parted forever.
The game has been long teasing that this relationship between them, as it stands now, means more to Nikki than she's letting on. She's a strong, proud woman and the she doesn't like feeling vulnerable, which is something that happens when you want to tell someone you love them. There's also the issue of them being brother and sister, and she will obviously be having reservations about not only how would he would react to hearing his sister confess that she loves him, but also how exactly they could have such a relationship going forward, especially while they're in college.
Deep down I think Nikki has greater feelings for Taylor than she lets on and, for all intents and purposes, this should mean that a romantic relationship is a possibility for them. But, if Sumodeine really means it when saying that they won't end up together, then something is going to happen to one of them that will keep them apart and it will likely be tragic.