Oh. I kinda guessed that was part of it too.
She seemed very insistent on getting him, while simultaneously cucking the brother.
In the end, we're still in a cuck square.
I don't think Kano is "cucking" the ML's brother. She just .....doesn't like him, but he's being super aggressively jealous and possessive over the idea of being the one to date her.
The younger sister's just an idiot who likes the brother and doesn't realize that he likes
her sister, nor does she realize that the ML likes her because he never confessed and now that he knows she likes
his brother he feels like he's bound to lose and has just given up hope.
It's not really that anyone's "cucking" anyone else, it's just a dumb unrequited love quadrangle where no one is actually telling anyone else that they like them.
Except, funnily enough, the younger sister, who sadly is ditzy and oblivious to the fact that despite being the most vocally proactive is destined to lose (unless the brother gets over himself and ..."settles" isn't the correct word, but gets with Suzuno since Kano wants nothing to do with him).