@Purplelibraryguy
Yes, Akio came later, after his first wife died and after he had Ma with Shiori's mom.
Tbh, she's an airhead, probably not very smart either - I wouldn't be surprised if she would just leave if the dad would say "you can leave if you hate Akio, but kids will stay with me" and since she was very young, she didn't care and didn't think of consequences. Also, he was a lawyer and as far we know she was jobless without any significant education. All of this, pieced together, kind of explains why she let go of her children so easily.
Let's not forget that both Ma and Shiori wanted nothing to do with her when she came back and preferred to stay with the dad, which suggests that she indeed abandoned them.
Basically, she was very young and stupid so she made terrible mistake while being controlled by her emotions (towards Akio). Of course, most of this are assumptions based of what we know so far (flashbacks and her current, apologetic behaviour which suggests that it was her fault).