Complex sentence: After Peters had returned to Russia in May 1917, following the February Revolution, and become deputy head of the Cheka, he invited his wife and daughter to join him there: upon arrival in Russia, they discovered that Peters had a new family.
Split into simpler sentences: After Peters had returned to Russia in May 1917, following the February Revolution, and became deputy head of the Cheka. Upon arrival in Russia, they discovered that Peters had a new family.