Aika is the main protagonist's older sister. She ended up moving back into their family home with her youngest child. She had a habit of letting her tits out everywhere and eventually one night while both of them were drunk he made his move.
Ane wa Yanmama Junyuu-chuu
This story about a married woman, as is usual in such stories sexually unsatisfied in married life, and decided to make up for lost time with her brother, who came to stay with her husband and children. What started as a harmless joke on a drunken bench, will grow into a constant frenzied sex always and everywhere, at any convenient opportunity. [MAL]
