Yes I agree the age gap is weird. But the wierdest part is he is her guardian.

It’s 5 years

u say that like it's normal when she's so young

Is he her guardian? I took it as less of a father's role and more of a literal guardian. As like it's his job to guard her.
Especially bc the women call her his "sister" not daughter.
The age gap is also only 5 years so when they met he was probably only 14/15 (just guessing based on her maybe being about 10 in the beginning, tho she could be 6-8 which would make him early teens)

And now that they're adults (there seems to be about a idk 6-8 maybe 10 year time skip? Just guessing bc they've not stated any ages) I don't see that big of a deal.
He never seemed to step into a fatherly role, more older brother/caretaker
And she has shown so far that she sees only her father as her father.

I can see why it would be weird but I just saw an slightly older kid protecting another. 5 years is a lot when you're young but they're seemingly both adults now. Not that big of a gap the older you get.