I am quite amazed about how quick Yuni's mom has caught on. The translation in the panel of p.18 or 188 raw, plus my own understanding are
Yuni's mom: "Your girlfriend did" (Kanojiuka?) Girlfriend?
Yuni: "......"(......) ......
Yuni "Nah, someone else"(Uun Chiu) Wrong
Yuni's mom "What was that pause just now?"(Nani ima no ma) What was that pause?
Yuni "Huh? I didn't..."(Ha? Betsu ni) Huh? Nothing.
Yuni's mom "Eh? Two timing?"(Aijin) Lover?
Yuni "Phrasing!" (iikata) Way to say it.
Must be intuition or personal experience, using both words girlfriend and lover in a conversation with her daughter, assuming that she has both. However she does not seem to imply Yuni is two timing (does something wrong).
You intuition is wrong and I'd appreciate it if you didn't invent your own stuff with your Google translate knowledge of Japanese and try to spread doubt or misinformation. You're not trying to translate manga I hope.
I was confused by your previous question about "lover", now I see why. You're using google translate or DeepL.
愛人 isn't a generic lover here. It's specifically in the meaning of mistress, or, if you want "the person I love sexually as opposed to the person I'm married to/in an official relationship with".
If she meant a "lover" it would be 恋人 (the one you love)
Here, I found a video that explains the difference: https://www.youtube.com/watch?v=lL0gvr2LOWk
Or here, an actual Japanese answering the question: https://hinative.com/questions/13485754 : 浮気相手 "Adulterous lover".
So, YES, she implies two-timing. Or her reaction, with the sparkling eye, doesn't makes sense.
And "chigau" doesn't only mean wrong, it means "different", so here it's a "no, different" (ie, not my girlfriend, someone else).
Are you Masina?