I almost think the sex was a dream. On page 6 after panel 3 there are some black bars showing the passage of time. They show up again on page 20 after which she acts kinda like nothing happened. It felt like a pretty abrupt turn from the previous pages. If I am right, and I hope I am not, she walked in on him masterbating, he kisses her, she acknowledges his feelings, he fantasies the next 14 pages of sex, she pretends nothing is weird the next morning. That is all I got to explain the epilog. I thought the cloth in her hand might mean something, but it is the apron she uses on page 2 panel 5.
Haruto-kun knows What Is Best In Life ... Kyouko-nee transitioning from "reliable Big Sister Childhood Friend" to a "Sexy Onee-San" girlfriend (under breath, Dat Lucky Bastard ... GRR)!
Protect that smile!