Complex sentence: It turns out to be Pyotr Stepanovich Verkhovensky, Stepan Trofimovich's son, and as he is talking Nikolai Stavrogin quietly enters.
Split into simpler sentences: It turns out to be Pyotr Stepanovich Verkhovensky, Stepan Trofimovich's son. As he is talking, Nikolai Stavrogin quietly enters.