I'm pretty sure she was just friends with his father, though I can't remember if it was said in-game or BD said it here. I'm also pretty sure that wouldn't make that much difference though, the law doesn't really recognise 'dating' as a thing. If she wasn't his actual stepmother, the care system in just about any country you care to name wouldn't distinguish between if she was dating his father or just friends. Interesting question though, she must have been very close for them to let her take him rather than him going into the system. You can't just call 'dibs' on a
kid. Or maybe someone was pulling strings? We know that
Valerie was closer to the family than we had previously realised.