We Haven't Changed

The protagonist came home to the countryside after a long time, and waiting for him there was his younger sister who now had the body of an adult.

This is a story about a pair of siblings who, after something or another find themselves taking a bath together, and then after this and that will end up having sex.

(Source: DLsite)

Note: the original, raw art features inverted nipples. Versions with regular nipples are redraws.