A big fat no from me on the marrying her part.
I typed out both paths explicitly for the inevitable over-serious crowd such as ^^this^^.
I'm assuming her character is merely exaggerated, showing her co-dependence.
But she's instead one of those "played for comedic effect" doji-ko / clumsy girls.
She's clearly an adult, as evidenced by her backstory in succubus world. And independence. (She's supposed to be hunting!)
I would argue she's not
literally the mind of a child in the body of an adult. Just pure. (That's what cute bonus "if" panels are for.)
Onii-san is just bringing out the (worst?) in her. Hence why I said "either". To avoid concern trolls. Guess it happened anyway.
And for the record, I find adult infantilism just as "a big fat no" as you find age discrepancy. It's played for laughs.