Complex sentence: When they doubt him, Helsing leads Seward with him at first to show him first-hand-first by showing that Lucy's coffin is empty, and then waiting when she comes with another child to feed on.
Split into simpler sentences: When they doubt him, Helsing leads Seward with him at first to show him first hand, first by showing that Lucy's coffin is empty. Then waiting when she comes with another child to feed on.