Iiwake mo Dekinai: Anekano

Kurata Yuna is a first year college student who has always been a foil to her beautiful and assertive older sister. When she goes out one day, she is roofied and raped at a hotel by a man she has just met. Later, she finds out that that man is her older sister's fiancé, with who both she and her sister will be living from now on.