Iiwake mo Dekinai: Anekare

Yuna Kurata 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 the man is her older sister's fiancé, with who both she and her sister will be living with from now on.

(Source: Shounen Gahousha, translated)