The Main Character is a straight up dumbass, why doesn't he call the police nor question why the sister is so obsessive over him clearly anyone with braincells would question why the sister is so obsessive. Also, also why THE HELL IS HE NOT QUESTION WHY IN THE FLYING FUCK IS SHE RAPING HIM.
Call police on his beloved? Man, MC is just playing tsundere. He is actually happy to be raped by her. The whole premise is dumb, they are not blood related relatives. She doesn't see him as a brother nor are they blood relatives, meaning they can actually get married. Their friend is actually doing what sister has told him, trying to create a fissure between them.
He himself stated that he is happy deep down and that he doesn't want to hurt his sister.
In essence if he didn't have stupidity of thinking of her as a sister (too late) he would already have mutual fuck fest with her.
I imagined to be him (applied a skill empathy). During ages of 5 or 6 for example suddenly a girl comes to live with me, i've already met other children in kindergarten by this point, basically now i have a "friend" living with me 24/7 now. I would not consider her as a sister fully EVER.
Miraculously instead of taking usual sibling rivalry as model of our relationship, i became a sis-con with my friend-sister.
She is a bombshell (sexy beauty) as MC i develop sexual attraction that i'm masking as strong brotherly love (it sure looks like it), why the hell would i call police, tell parents or anything else?
But in order to create a stupid drama he will tell his childhood friend who will have a power struggle with the sister.....Childhood friend i think also recognizes the fact that sister isn't blood related and in order to separate them plants ideas in to this stupid MC head in order to separate them. Sister is obsessive thus it get's ugly, because despite cultivating strong relationship with him over many years he was easily swayed by some words from another girl, she tried to be reasonable but failed.
MC is a moron in this story through and through. If he didn't like her i would understand your statements but he clearly likes her, he just pretends not to like her as a woman because he is ashamed to admit it.