Marianne was astonished to discover how she and her mother had forgotten such considerations as Edward's mother and sister. A little odd. Marianne was astonished to discover how she and her mother had forgotten something. This part is pretty easy Marianne and her mother had forgotten something and she was astonished by that. The "something" that has been forgotten is "such considerations as Edward's mother and sister. " The use of considerations is a little old fashioned but it would appear that Marianne and her mother have a perceived responsibility for two people, the mother and sister of Edward.