*I DONT MEANT AN ACTUAL COUSIN* I mean when your parent has a best friend and you grow up calling them aunt or uncle. Then they have a kid and you call them cousin even when you're not actually cousins. What if you were to date or do things with this "cousin" Would that be weird?
No but it is weird to call someone like that your cousin. You do realize for someone to be your cousin, they actually have to be related right? It's a family friend, not a cousin and yes its perfectly OK to date a family friend.
In a lot of Islamic cultures they also all marry their first cousins so go figure.
If they like you then go ahead and date, like you said their not your real cousin, but if they see you as a real cousin (in their mind) then don't cross that burning bridge, I report don't cross it. Good luck pal.