I didn't see Step Brothers this weekend, but one of our readers did. And he mentions there's a big a cappella moment in the film.
The San Francisco Chronicle review confirms this, and says a cappella is a bright spot in the movie! The review reads: "We meet Derek in his car, on the way to his mother's house. He and his family are doing a regimented a cappella version of 'Sweet Child O' Mine' by Guns N' Roses, and he almost gets in a car accident yelling at his wife for her bad pitch. It's hard to explain, but inspired. If only more of the film had featured his snarling face."
We couldn't find the exact clip, unfortunately. But here's the Binghamtonics singing "Sweet Child O' Mine." I imagine it sounded a lot like this in Step Brothers.