Complex sentence: Charles rescued his mother by pushing her out of an upstairs window, but was unable to help his brothers, two of whom were in the same bedroom as him; opening the window had caused a draught which fed the fire.
Split into simpler sentences: Charles rescued his mother by pushing her out of an upstairs window, but was unable to help his brothers. Both were in the same bedroom as he; however, opening the bedroom window had caused a draught which fed the fire.