Yeah, I understand sister would make more sense, but I don't really want to change the message from the original author, the mangaka is calling their relationship father and daughter; plus we made the english title like so so that this manga can be searched more easily on the internet since it's barely new and not much is known about it on google!
The parent-child in question could've been his father and him, since he's succeeding his father's weapon shop, and not of him and the girl (I'm aware it's using father-daughter kanji but I don't think there's parent-son in kanji, it's either father/mother/neutral parent + daughter or neutral both with 親子)
Also if the goal is to make it easier to search it's not working , I tried copying the current EN title into google and I get completely unrelated results.
Even adding mangadex to the requirement on top of this series I also get
https://mangadex.org/title/3299fab7...-an-uncle-who-runs-a-weapon-shop?tab=chapters as an extra (which, who'da thunk, features a man and a small girl running a weapon shop...)
Also you ALREADY changed the message of the author by giving it a new, more generic title.