I thought this was obvious speculation fuel as well (though I didn't add it to the pool).
However I just noticed something very obvious that would lead to them both making those expressions.
If you look in the center of the picture you'll see that their hair braids have become intertwined. Sakuya is slightly annoyed by it whereas Meiling is very happy about it.
I'm sure a romantic theme began playing in the background the moment this happened.
Sakuya caught a case of the slackies from Meiling and fell a sleep right beside her. Marisa then walks by and ties their braids together as a joke and then heads off to "borrow" some books from Patchy. Later Sakuya wakes up and we have our current situation.