What is the difference between in-laws and outlaws?

I was "Mexican Married" to a guy for 20 years. To those who don't know, that's being together, living together over 5 years. The relatives all just call you married. So I told his sister she isn't my sister-in-law. She had shocked Pikachu face. I told her we aren't legally married, so you're my sister outlaw! She loved it!