Jerry O'Connell who plays Georgie Cooper is supposed to be Sheldon's older brother by several years. In real life, O'Connell is actually about 1 year younger than Jim Parsons who plays Sheldon.
Jerry O'Connell, who guest stars as Sheldon's not so bright brother George "Georgie" Cooper Jr., once starred in his own show Sliders (1995), as boy genius Quinn Mallory who was also a string theorist, somewhat like Sheldon.
Jerry O'Connell was once indirectly referenced on the show in The Russian Rocket Reaction (2011) during one of Sheldon's rants against Wil Wheaton, where he called Wil "the one kid from Stand by Me (1986) that no one remembers." O'Connell played Vern in that movie, one of the four child protagonists.
Raj mentions his brother Adoot, so between him and his sister Priya Koothrappali, two of Raj's five siblings have now been mentioned by name in the series. The names of his remaining two brothers and one sister are never revealed.
When Sheldon warns Leonard not to fall for George's "Nerd says what?" joke, Leonard still falls for it. This is a sort of poetic justice, as Leonard used the same joke himself on Penny's friend Kurt in The Middle Earth Paradigm (2007) ("homo habilis man discovering his opposable thumb says what?").