Vin was more confused regarding their relationship since she was also crushing on him. Kelsier, naturally, never notices. But Vin does say that Kelsier was like a father to her.
And as of Secret History Kelsier is straight up calling her his daughter, so that annotation is a bit outdated. A later one says Kelsier is dead and not coming back, so...
This isn’t theory so much as a difference of opinion on what constitutes a parent-child relationship. You probably wouldn’t consider my aunt my aunt, for example.
Older adoptees are less likely to acknowledge their adoptive parents as their parents for a variety of reasons. The parents, however, usually acknowledge the kids.
Had I ended up with a third sister, I doubt she would have ever considered my mother ‘hers’ the same way she did her bio mom. (Who was a great person, btw. She had a psychotic break after her husband up and left.) But my mother would have considered her wholly her daughter. I know her actual adoptive mom does. And if certainly count her as a child of both her biological and adoptive moms.
Back to the game:
This character is known for their stories.