What is the name of episode when simon sick on carlys brother in inbetweeners?

You're likely thinking of the episode "Carls's Brother" from the Inbetweeners. It's the ninth episode of the first season.

Simon does get sick on Will (Carls's brother), but it's not due to intentional sickness. He has a panic attack in the toilet, vomits, and it splashes onto Will. It's a pretty iconic scene!