pseudocousin 

Your parent's best friend's kid that you are very close with and consider family.

Guy 1: I was hangin' with my cousin last night... well, my dad's friends' kid.

Guy 2: Oh, your pseudocousin?

pseudocousin by TheStig1214 July 10, 2012

pseudo-cousin 

a person whose family has known your family for an extremely long time and whose history you know so well you might as well be related (even though you technically aren't).

Amy's parents grew up with her best friend Kate's parents, so Kate and Amy are pseudo-cousins.

pseudo-cousin by busrider December 27, 2008