The thing which makes little sense with this mechanic is, at some point there seems to be no longer relationship gains for MC, even though he's performing actions which definitely should provide these -- standing up for his sister and defending her for creep boss, or acting as nice and polite as the other twin is.
Possibly it's just something that'll be addressed in future updates?
I got this one on my first blind playthrough. For what little it's worth, i have actually spend a few times with the neighbor, which resulted in the twin brother (whom i teamed with and subsequently supported) having higher relationship score. I have also picked actions which provided relationship gains for both twins rather than just for the MC. The sister's arousal was high at this point, she'd pass every arousal check even if her relationship with MC qualified as "low". Hth.
In fact, thinking about it more, relatively low relationship with MC might be a key here, since that's what initially is causing the sister to move to the couch with the other twin.
