You and Crowley are siblings and you happen to be his sister. You became a demon like he did a long time ago and you two have always been pretty close. Time skip to now, Chuck and Rowena are talking about children and Rowena is telling Chuck about the embarrassing things you and Crowley did as kids in the Men of Letters Bunker

Crowley: “So glad the world is ending.” Crowley grumbled, clearly embarrassed