Do S Ane to Boku no Hounyou Kankei

Kei is living together with his elder sister, Rei. Their parents are working abroad so they are left at home, just the two of them. But instead of helping each other, Rei just bullies her younger brother, passing all the household chores to him and doing other nasty stuff when she's in a bad mood. She even bullies him at school (by taking away his lunch for example). Kei, just like the nice guy he is, however never complains and accepts all of the cruel things done to him. But all of this is about to change. One day, a foreigner girl starts attending their school as a new transfer student. She gets along well with Kei like she's known him long before.