She squirms a little at the word 'lucky.' Lissa knows he's just saying that because that's what you say when you hear someone is getting married, but... well... maybe that's the problem. It's a form response, not knowing anything about the situation. Unless he does know who her brother is-- then it takes on all kinds of double and triple meanings. Maybe. Or maybe not; he already knows Bunta and presumably that means he knows what Bunta does for a living. Maybe he knows already that Bunta and Seiichi are personal friends and it's a sincere exclamation, not just the done thing.

She squishes her cheeks between her hands just to make herself think about something else.

Luckily, she's surrounded by lovely things to think about.