Sheldon is shown doing his three knocks routine that he did on The Big Bang Theory (2007). However, during "The Big Bang Theory" he had stated that he implemented that habit as a teenager, after walking in on his dad in a compromising position with someone other than his mom. EDIT: But this knock isn't his "The Big Bang Theory" repetitive three-knocks-and-a-name. In this instance, he knocks three times and says "Mom", then he knocks three more times and says "Dad". He doesn't repeat either sequence at all, so, while similar, it's not the same.
Sheldon has an IQ of 180+ and would never have used the incorrect phrase "me and my sister," as they have him do in the episode. He would have said the correct "my sister and I."