Seen this theme before in a classic work by Kamino Ryu-Ya (was probably here in the days B.L. - Before FAKKU want Legit). Male protagonist gradually hooks up with three sisters, middle sister is rather (if not seriously) tsundere, and ultimately is the No. 1 girl in the harem, after the usual fights, quarrels and everyone sorting out their real feelings.
I'm looking forward to him having sex with the stern black haired sister as she (like in most such plots) most likely loves the boy the most, but thinking that the prefers her sisters acts cold so that she won't get hurt. I would say she will be forced into a situation by her sisters which will lead to her being honest with her feelings and switching her lust into overdrive.
Fingers crossed for a pregnancy ending...
Sounds like some great fun in summer indeed...I'd love to see more of this fun in possible sequels, especially since Kana didn't even appear for that long yet.
So first he'd go through them one by one until he has done it with all 3, followed by a foursome and then the ending, I suppose?
Definitely needs a sequel, multiple sequels, in fact.