Well uh, this is rather embarrassing. Mother-in-Law is indeed a translation mistake. Hax confused 故 with 姑. The former means "consequently", and the latter means "mother-in-law". This got past me during my 2pass because I usually only retranslate a line if I feel like something's wrong. And since I did the check after I just woke up, my brain was rather sluggish and never thought "Hey, mother-in-law doesn't make sense cuz she's not fucking married yet". So in essence, this was a result of Hax being bad at life (I'm gonna give him so much shit for this lmao) and me just being dumb. I'll include a note about it in the next chapter's endpage as well.