Please note something: "義理の" (giri no) generally means "a relationship through marriage" and thus "義理の妹" can mean both "a younger stepsister" or "a younger sister-in-law". I think it's supposed to mean the former here and not the latter.
Waaaah!There, there.Onii-chan, let's go to school!It huuuurts!We weren't related by blood but, before I knew it, she became my precious, true "little sister" in my heart.Mao-chan was a crybaby so I couldn't leave her alone,I was happy to be a "big brother". So I had to take care of her....but...Mao-chan, the little stepsister I got when I was in 3rd grade.
