I thought the same thing. For a sec I thought she figured it out after seeing those two interact with each other, but then she asked Misaki if he went to the theme park with his gf >< Pretty much everyone on Usagi-san's side knows, I wonder who will be the first one to figure it out on Misaki's side...
Thinking about it now, her asking about the girlfriend might actually be a way to confirm the declaration Usagi made of "knowing something is precious before losing it". He also nodded to her as if saying "you're intuition is right". At least she noticed Misaki's reluctance of leaving Usagi's side. Can't wait for what's to come ~
I really wanted Misaki's sister-in-law to realize what was up and am kind of sad she didn't. Women's intuition and all. Holding out hope that this does happen though!