As someone who has a sister, I really agree with Torii-kun's friend about the jealousy thing... It's not like her sister loves her in a lustful way, its more like being concerned and being watchful. My sister usually hates it when I'm with my girlfriend because she thinks she might end up hurting my feelings, smth like that...
... But then again, this is Noise we're talking about. Anything is possible with hentai.