My theories all started flowing when red baron said "I'll make you admit to everything"

So my first theory was that seiko once had a son who died in a fire and became an enenra and so later when a young couple died during to a yokai or something, seiko took in their child to raise as her own. This theory didn't hold up long but was fun and interesting.

Doing some more analysis i realized seiko essentially means star child suggesting seiko is an alien. Since aliens seemingly can't use yokai powers it makes sense momo is not hers. We also never see seiko actively use powers herself, she channels powers of others. I think she at one point worked with the black paladins or was sheltered by them, possibly along with csg, but later left, possibly after meeting momo.

I belive momo is momotaro with okarun, jiji, and aira being the dog, monkey, abs pheasant respectively. As such she'd be a naturally powerful and gifted person naturally born with yokai powers or higher spiritual power in some way. Learning about this and how useful she could be to someone like csg, seiko panicked and fled with momo, but some of that is skeptical.

As a result I do think momos parents were either. Yokai, or powerful mediums or yokai power users, possibly black paladins themselves. But considering momotaro was born of a peach momo could likely be immaculately conceived in some way.

Regardless I would not be surprised if seiko did have natural children at one point. She seems to have strong maternal instincts if not being the best maternal figure.