Thomas - 19 years old, currently in the US Navy
Chris - 23 years old, currently employed
Katie - 15 years old, currently in high school
Emily - 15 years old, currently in high school
Matt and Emily are brother and sister. Chris, Thomas, and Katie are also siblings. These two sets of siblings are not related by blood, only by marriage. Matt and Emily's father's brother's wife's brother's kids are Chris, Thomas, and Katie. Or you could look at it the other way; Chris, Thomas, and Katie's father's sister's husband's brother's kids are Matt and Emily. Got it? Good, now work this into it: Matt and Emily's mother and Chris, Thomas, and Katie's mother were best friends growing up.
End result: we grew up with a cousin-like relationship with each other, even if the word doesn't technically apply. We call each other cousins, and that's what counts.
... More than you wanted to know, we're sure.