What makes step brothers so good, is that it's a parody of most comedy movies. The last part of the movie is basically a 15-20 parody of how most comedies feel the need to cram an emotional/mature moment into the last act of the movie, despite it often making no sense in the plot. When Brennan starts singing at the wine mixer, everybody has a emotional moment spurred by his voice and suddenly all is well, all of these horribly adjusted people are suddenly loving and caring folks at the drop of a hat.
I think that movie went way over peoples heads, the director/screen writer (adam mckay) is one of the goats