User blog comment:B214/High School DxD DX.4 discussion/@comment-30924344-20170721201027/@comment-2601:8C:601:5692:3D30:770C:CE5:1085-20170725150359

Of course, due to age, it might be the siblings caring for grandma instead. It might have been that they were going to be separated due to her being deemed unfit as a caretaker, thus leading to Sona's intervention (and that might have been when/why the sister learned of her older brother's reincarnation).