Okay but are we going to skip over how the maid is seventeen?! I mean I guess in comparison to having an adult in a child's body make a child promise to marry her sure why not, but still... 17 year olds are teenagers, so
A. long term decision making skills aren't to be trusted and
B. why the fuck would the grandpa age dude want to be with someone that damn young, I'm 27 and I'd feel like a hell of a creep if I was in a relationship with a 17 year old.
I get this is a medieval-ish setting but even so, the MC isn't from that time she's from our time so why is she so dead set on the grandpa being cool with it? For that matter, she doesn't seem to care about his feelings, just wants him to accept the maid's.