Sorry, but
acewinz is probably correct here.
Look at this:
View attachment 539192
Based on the dialogue, the intended relationship is most probably Father/Daughter, as the dialogue becomes very....odd if you change it to basically anything else.
It is a little odd to say "Hi boyfriend, [insert more dialogue here]" or even "Hi brother, [insert more dialogue here]".
But it does make sense to say "Hi dad, [insert more dialogue here]".
The MC is older than her for sure, I dont know how you got the age of 32 or younger, but I dont think its mentioned anywhere in the game.
Even so, she could be 18 and the MC could be 36. Or you know, she could be 16 and the MC could be 32....but those ages are not going to be mentioned in the game explicitly like that, for obvious reasons.
There are many other instances in the game where she goes "you're my [insert what she calls you here] !".
The MC also goes like "I have a [insert what you call her], you know."
Now look at the following examples:
"you're my dad!"
"you're my brother!"
"you're my boyfriend!"
and
"I have a daughter, you know."
"I have a sister, you know."
"I have a girlfriend, you know."
If you put it all together, they are father and daughter. Everything else starts to seem odd, the way the things are phrased in the game.
The dialogue just makes more sense that way.
Having said that, if you change the relationship to a pet name, something like "Cutie" and "Tiger", there are some hilarious results, I'll tell you that

And yes I did that for the lolz.