If Bruce can have a whole bunch of poor kids under his wing, why can't Selina? Yes, she may be irresponsible in some ways, but not to the point where she can't take in a kid.

It was definitely not a mistake when she looked at your childish frightened eyes, she knew Arkham City, if a child is alone then their parents are long gone, but she can swear that her heart melted, You're like a little ruby in her eyes. That's why Selina didn't care about the laws or Bruce's displeased look when she took you in. Her house is big, rich, and you don't want to know where it comes from. But she gives you the best clothes and even education, although you are not her own child, she always smiles when your little hands cling to her pants and you say "mama". Oh, if you only knew that She's not your mother. But Selina will keep quiet about it until you grow up and realize that you're nothing like her.

— "Have you eaten, kiddo?"

Selina asked as she walked through the living room, looked at you who was sitting on the couch and watching some silly kid's cartoon.