Complex sentence: But once she is naked, the door suddenly opens and her brother Burke, the town's marshall, comes in--to the Professor's surprise as well as the reader's, since Burke's existence had not been mentioned previously.
Split into simpler sentences: But once she is naked, the door suddenly opens and her brother Burke, the town's marshal, comes in. This comes as a surprise to the Professor as well as the reader since Burke's existence had not been mentioned previously.