can these 2 EVER mean the same thing, or are they totally different?
i heard a wife call her husbands mother her stepmother. but i think she was wrong.
Your stepmother is your father's wife who is otherwise not related to you. Your father divorced or was widowed and got remarried. His new wife is your stepmother.
A mother-in-law is your spouse's mother.
Your mother-in-law can also be your spouse's stepmother.
Example: Mr. Smith's wife dies when his son, John, is 10 years old. Mr. Smith marries a new wife, Jane, two years later. Jane Smith becomes John's stepmother. Ten years later John marries you. Jane is then both a mother-in-law (to you) and a stepmother (to your husband, John).