Complex sentence: Brian realizes that Stewie is right and decides to listen to his advice; however, he finds out that Carolyn has began to date Cleveland, since Brian did not want to have sex for the three weeks they have dated.
Split into simpler sentences: Brian realizes that Stewie is right and decides to listen to his advice. However, he finds out that Carolyn has began to date Cleveland, since Brian did not want to have sex for the three weeks they have dated.