This could have been played a lot darker than it did. Imagine having existential power over another person. Your choice decides if they disappear or can stay. What a crushing pressure.
I would say readers don't care about realism per se. It's about plausability, things can be as out there as possible as long as the events feel plausible according to the created worlds rules.
She helped ease his loneliness. I dont think he'll care too much about not having any blood ties. Technically she hasn't lied about being his granddaughter. It's all been his misunderstanding, and she hasn't corrected him yet.