The setup makes sense to me when I fill in the blanks with the likely situation. Sheldon's sister seems unmarried or she married a military man who is currently stationed far away as there is no mention of the father at all. Mom would see this as a family situation and require Sheldon to be in attendance, regardless of Sheldon's or his sister's objections. One thing we know from past interactions is that Sheldon's mom always gets her way. I thought Sheldon's comments were closer to Sheldon of the past than his newer and meaner self. However, this seemed like a perfect opportunity to shift Sheldon towards possibly being a father and getting the first inkling of the idea that there are things and people in life more important than yourself. Like having him amazed at holding the baby but in Sheldon fashion, seeing it as an opportunity to create/raise the smartest child on the planet as if it was his pet. This could then be used to move Shamy's relationship forward. Alas, it appears the writers are not going to move Sheldon in that direction but continue to keeps Sheldon completely self-centered and mean. Too bad.