I also read your query on the CA forum. Have you had any contact with your mom since the divorce? She most likely has a new surname, as I'm sure she probably remarried, so it's difficult to find someone, especially if they don't want to be found. All I found: Birthdates/locations for you and your sister; marriage/divorce dates for your parents; deathdate/location for your dad.
I might add, her name was Diana on birth/divorce records.