Which tenant was the mistake in The westing game?

Judge J.J. Ford says SyDELLe Pulaski is the mistake, because when Sandy McSouthers and her were investigating, they received a letter from Sybell Pulaski, Crow's school friend. Sam was suppose to give a room to Sybell Pulaski, but instead he gave it to SyDELLe Pulaski.