Omae Imouto ja Nakute Iinazuke datta no kayo!? - Vol. 3 Ch. 13

[Hapid]

He is definetily not dense. Just look at previous chapter till now. Its just suddenly engagement from their parent and the walls between siblings makes him not to think that way

I mean, just imagine after all of this year, and your parent suddenly tells you that your little sister is not related, plus are engaged to her