Or it could be that since she lived under the same roof he forbid her from training with weapons. It seems she's essentially adopted, but without actually being so.
Viewing post in Hero's Harem Guild (NSFW +18) comments
Could be, allright. But Friend? Caretaker? Come on, what are the chances?
Plus there's Patreon and other sites ban on incest that makes it likely that she was supposed to be the sister - and the mother, respectively - and changed into what she - /they - are now described as. While incest isn't one of my kink i'd rather play the game as it was intended.