Ok kiddo, you know how you have brothers and sisters? Well sometimes mommy or daddy has brothers or sisters too, and when they get married, those brothers or sisters become your sibling-in-law!
For example, if mommy has a brother named Uncle Bob and he gets married to Aunt Sue, she becomes your new Aunt Sue and your sibling-in-law because she married your mommy's brother.
Just like your real siblings, you can have a good relationship with your sibling-in-law and play games and have fun together. But sometimes you might not get along very well and that's okay too. Just remember to always be kind and respectful because family is important!