Even if there's no incest going on, I still think being friends to get close on his sister is dishonest. Setting the stealing aside, you're not being a genuine friend to him.
Why does it feel like no one understands how perspective works. Ofc the mf isn't a nice guy. All he's done is think about how much of a better "match" an Idol would be for his sister that's also an Idol.
I'm pretty certain he's talking about being a "good fit" for her and not in the most general of senses.