When Sebastian gets volunteered to have to go spend time at the old folks home, he raises his finger in the shape of a gun, right? Now rewind that and watch him VERY closely, you will see that he is a little off cue and raises his hand at the wrong time, and then he corrects himself!
It COULD be a slip-up, but I took it as Sebastian was going to do it but got interrupted when Aunt Helen hugged him. It also could be that the woman who played Aunt Helen wasn't supposed to hug him, but she did as an improv.