I would like to see a study on that because thats hard for me to believe, but what do I know. I don't think when your sister says 2 week memory she is referring to your baby remembering you. I don't think our babies forget us or their other companions. When I leave for 2 days my in-laws tell me Georgie jumps everytime he hears a car. I don't think you should worry he won't forget you, I am sure you have left too much of an imprint on his life
Oh and I have anxiety over leaving Georgie for an evening at dinner so I know how you feel completely