Ane no Musume (Pre-serialization)

Her trust broken by her girlfriend, Saya moves out of the room they were living in, and, with nowhere else to go, she has no choice but to stay with her sister's family. There, she reunites with her sister's daughter, Rui, who she often used to play with when she was younger. Rui used to be such an innocent girl, but now she's almost like succubus meant to lead people astray, boasting calculating smiles and charm unbefitting of her age. Day after day Saya is filled with confusion and doubt, her resolve gradually wavering away due to Rui's intimate sense of distance, until eventually she loses sight of any reason to turn Rui down...

My sister's daughter has grown up into a little devil - After being cheated on by her ex-girlfriend, and was kicked out, Saya moves in with her older sister and her husband. However, Saya is seduced by her sister's daughter, Rui, and kissing becomes a daily routine.

An anthology organised by Canno and released for Comiket 100 The tagline was "Sweet and happy love stories featuring 3 people or more."