I agree, Masumi is at fault for always saying "yes" to Satoko and giving in, and as you said she is clearly doesn't understand the emotional impact which she is creating for herself. Let's ignore the incest part, for now, it is pretty obvious that the relationship will have consequences. However Satoko really hit the nail in her coffin here, was this rape? yes obviously, will this give trauma to both of them and mostly Masumi, yep of course. It doesn't matter if you are in a relationship, if a person agrees to have sex with you then says "I'm sorry I'm not feeling like it" you step down and say "okay, no problem" but this clearly was rape, it didn't have consent. Satoko is an adult, if a person says no, it is a no but she went on it saying "why were you saying no, look at you, you clearly were enjoying it". What in the actual hell was that answer?. Sure she understood her actions when Masumi started to cry but what will "sorry" do for now? Satoko was being forceful and maybe emotionally manipulative towards her own aunt. Actually, in the first chapter, she didn't care if her aunt said "we can't do this, it isn't right", she just had her sad puppy eyes and Masumi gave in. Going back at the incest part, realistically speaking this relationship wouldn't work and clearly, consequences would be brought on the matter. Imagine if her own mother finds out that her daughter is sleeping with her sister and that she raped her too, clearly the family would bein ruin.

Bro, jesus christ chapter 5 was a depressing read. Nekozaki looks so emotional defeated, poor thing almost had a breakdown while trying to confess her emotions to Mikoto only for her brain to take the worst of situation, now she is thinking that "Yes of course she see's me as a friend". I don't dislike this even if it dosen't have a drama tag, it is realistic. I have a feelibg that she might have been abused or such since Nekozaki seem's like she is so starved out of love, while Mikato also tries not to go to deep out of fear that Nekozaki won't like it. Ah i'm big sad