Analyze the relationship between 'St George Bank' and 'Westpac' within the context of the input sentence.
Options: stock exchange, location of formation, position held, parent organization

Mrs Kelly was chief executive and managing director of St George Bank from 2002 to 2007 and CEO of Westpac from 2008 until early this year, overseeing a merger of the two banks in 2008.
parent organization