He healed her loneliness

Yuki Shiromiya was a normal sweet girl who didn't have that many friends so she was always lonely until she met her true love Kazuo Yamada, she'd fallen in love with him at first sight and knew he was the one for her. Shiromiya gets jealous when she sees other girls getting close with Yamada, she thought of getting rid of the others so she could be closer to him and tell him how she feels.
Her brother Osamu is just like her and together they will do anything to keep their lovers to themselves and not let anyone else have them.