Keiyaku Shimai - Vol. 3 Ch. 16 - A younger sister who loves her older sister

[Ravenna-rei]

Their mother didn't walk out on them: she is dead! (Before that, the most likely scenario is that they got divorced, and she got custody of Meguru while Hiromi went with her father.) In chapter 14, you can see someone (probably the father) burning incense at the family altar, with her portrait displayed. Her mother's death is why Meguru was left in her father's care again.

It is still a good question why the girls were raised separately.

Kinda hope it's retconned or something. Perhaps the dad is being performative and she's been divorced and lost all custody.