Complex sentence: When Serena was around, Blair was always left in her shadow, so when she returned Blair was not particularly thrilled to see her, especially when Blair finds out that Serena and Nate had sex the summer before Serena left.
Split into simpler sentences: When Serena was around, Blair was always left in her shadow, so when she returned Blair was not particularly thrilled to see her. Especially when Blair finds out that Serena and Nate had sex the summer before Serena left.