Complex sentence: The pair appear in his life once again when River creates an alternate reality by refusing to kill the Doctor, and witness the pair marry in the alternate universe before returning to the established course of events where the Doctor once again appears to die.
Split into simpler sentences: The pair appear in his life once again when River creates an alternate reality by refusing to kill the Doctor. They witness the Doctor marry River in the alternate universe before returning to the established course of events where the Doctor once again appears to die.