This is from Charlie (Rin's sister) history. We can tweak it if we want. But at least this will be an overview.
She is Rin's younger sister. As Rin's father died when she was only 3 year Charlie has a different father. Rin's and Charlie's mother remarried 6 years after Rin's(when she was 9) father died. The marriage was never a good one though. The real reason behind Henry and Piper(Rin's & Charlie's mom) getting married was because Piper got pregnant with Charlie. Almost immediately Henry started abusing Piper and Rin. Rin hated Henry but tried to respect him since he was her step father. Rin was always very protective over her little sister and would stand between her and Henry on a regular bases, and would get abused even more because of it. Henry cared for Charlie much more than Rin and frankly hated her. He would spoil Charlie and treat her like a princess. So Charlie was always the center of attention and that made Rin a bit spiteful of her younger sister. About 10 years later Henry had started abusing Charlie and was even worse to Piper than ever. He went missing shortly there after to never be seen again, but he wasn't really missed.