This is a definite downside of calling people by their last names; you'll run into problems when you meet their families.
Of course, Yukari does sometimes call Miho "Miho-san" whenever other members of her family are around or come up, so it's possible she's anticipated situations like this.
I did this when I visited my friend's house. all five of them looked at me at the same time and asked "what is it"? we burst out laughing after a second.
miho is confused!! in her confusion she did not attack!!
All Star EnsembleOh no, it's the full "Nishizumi-dono" line up!?Ummm... I meant the "Young and Lovely Nishizumi-dono"~!Nishizumi-dono~!ZUMI!NISHIWhy!?