Sif didn't actually know any of these people but Loki and Thor, except vaguely on the network, and she hadn't known Loki told Tony he was bringing a date, either. (Then again, she didn't know Loki had a girlfriend, so she didn't know who she was being mistaken for.)
So her response was probably kind of obvious to the two of them who had met her.
First she rolled her eyes at Thor and Loki, and then gave everyone else smiles and a, "Hi," and then immediately crouched down by the dog and offered her fist. "Hiiii, puppy. Hey there. How's it goin'?"