I really liked the angle myself, siblings loving each other but kept apart by social taboo so both in denial and madly repressing as they both jump into seriously flawed relationships to escape themselves. Is quite interesting the transformation from not giving a damn about what others thought to nearly every dialogue towards end revolved about what others might think. Yes it would have been nice to have had Nikki and Taylor together and who knows what happens in sequel, maybe like there Mother they get a chance at their forbidden love later in life.
I'm going to refence another game here..sorry, but the parralel between the two games are too similar.
Compare this game to Haley's Story:
In Haley's story, there is no doubt that Haley's always been in love with her brother. Since her early childhood, everything she did and still does centres around her brother. Every relationship she had in the past, had her brother in mind. She tried her best to hide that feeling, but is so obsessed with her brother that she is fully unable to do so. The events that happens in the game (in the canon version) is basically her most secret dream becoming true.
By contrast in My sister My Roommate:
Nikki is completely different then Haley. She is very popular. Never had a crush on her brother in the past as far as I remember. Even when they were roomates, she started having sex with various guys, not really bothered about her brother. At some stage, she feels that her brother is wasting his college years away by never going out and never getting laid. Nikki decided to do something about this by trying to get him into relationships and loosening him up by walking naked in front of him etc. etc.
Yes she also pushes her brother into a relationship, but the reasons are totally different. Haley does this to hide/supress her own feelings about her brother. Where as Nikki genuinely wants her brother to be more social and outgoing. The fact that Ashley was a complete nut job, was not her fault.
Then Nikki, being Nikki (Enjoying life and does not always thing about the consequences) started to push the boundries and eventualy had sex with her brother. Here I will agree. In my view, Nikki did fall for Tales, but this is not a childhood fantasy coming true (Unlike Haley). This is a spur of the moment thing that grew into something she was not ready for. Like every normal relationship, one can get over it and move on. That is what Nikki and Tales did.
Yes, I think she will always fondly remember her fling with her brother, yes there were still a lot of sexual tension left between them, especially from Nikki's side, but no, she will be able to move on and her choice of Marc is precisely because of her fling with Taylor. Marc is similar to Taylor on all accounts.
By contrast, I think Haley will never be happy without her brother. The only way she gives him up willingly, is by sacrificing her own love and accepting that it can not work out. But she will not live a happy life without him.