Complex sentence: He marries and fathers two children, but his wife is unfaithful and he eventually forms a relationship with Sebastian's younger sister Julia, who by that time has married but separated from the wealthy but coarse Canadian entrepreneur, Rex Mottram.
Split into simpler sentences: He marries and fathers two children, but his wife is unfaithful and he eventually forms a relationship with Sebastian's younger sister Julia. Julia has married but separated from the wealthy but coarse Canadian entrepreneur, Rex Mottram.