Sister Breeder

Shouta is very affectionate towards his sisters Rena and Aina, they grew up as close siblings. Day by day, he becomes more affectionate towards Aina... and eventually he even sneaks into her room at night!? Rena tries to stop Shouta, but ends up receiving his affection in Aina's place. Due to her hidden feelings for Shouta and the pleasure of experiencing things for the first time, the once innocent Rena is steadily being trained by Shouta...!?

(Source: Lune-soft, translated and edited)