well is not like her sister is the only meguru in town, like i get that i´d be weird to hear your sister s name while being hugged half naked but assuming that her sister is your co-worker s ex based on just that would be reaching a bit too much
To be fair we don't know her reasoning yet, could've put 2 and 2 together given meguru's age/vocally stated opinions on intimacy/approximate body type. might not even truly suspect anything but is finds the homonymity noteworthy or worth investigating. her immediate reaction's subdued enough that it could be any level of "huh!" plausibly. (Though i hope they do confirm it sooner than later, though i guess finding it out from all 3 running into each other at once could be juicy enough in its own right dramas-wise...)
I'm internally battling over whether to leave a pithy comment about this love triangle being Soooo Weird (which, like, it is, it isn't a triangle even setting aside the layers of simulacrum going on, and it isn't even "love" for all hypotenuses, etc.) or to leave something deeper than I'm really prepared to abt how this is really managing to capture shame surrounding sexuality (and the different causes OF this shame and what this shame in itself causes, tip-toeing around invoking homophobia directly though, people in the audience for this sorta yuri drama probably don't need that re-iterated :p) while still being pretty contrived from a, like, causal perspective. Thankfully the characters & motives are strong enough that the "coworker just so happens to be new step-sister's ex girlfriend & just so happens to have a fetish complimenting ones own" thing is still suspendable disbelief-wise.