Many of the people I know use "step-mom" only for a lady who lived with them when they were growing up and acted as a mother to them. They use "My father's wife" for the woman their father married after they moved out. Note that is "many", not all.
Your step-mom's sister would be your step-aunt, or "my father's wife's sister" or "Matilda" or "Ms. Smith", depending on how well you knew her, how well you liked her and what she prefers.
If the lady in question died 100 years ago and you are talking about genealogy - "Mary Todd Lincoln was my 2nd great grandfather's step-aunt" then "step-aunt" is an accurate term, no matter how late in life she married.