oh that can easily be explained
it's just a creative thing that the mangaka did to show that while Miku refers to Yukiya's mother as "okaa-san", i suppose what she really means is like "mother-in-law"
it's not uncommon for kanji to straight up be changed like that lol (idk how else to describe it, assigned?)
though, really I think I should've added a note on that page or something but idk how i would go about it in a way that's concise and is meaningful, like "oh she's just calling him her mother but she really means mother-in-law" or something??
if you don't agree with what I did, that's fine, I just tried to make a compromise