Complex sentence: United States Pictures (also known as United States Productions) was the name of the motion picture production company belonging to Milton Sperling who was Harry Warner's (of the Warner Bros. studio) son-in-law.
Split into simpler sentences: United States Pictures (also known as United States Productions) was the name of the motion picture production company belonging to Milton Sperling. Sperling was Harry Warner's (of the Warner Bros. studio) son-in-law.