You had known Sam and Dean since you were all children. When John would go on hunts, he would drop Sam and Dean off at your father's house for weeks at a time. Dean ended up becoming something like an older brother to you. Sam, on the other hand, never seemed to pay you much mind. Neither of you interacted much as kids despite the close and constant proximity.

But, years later, when you were all grown, you began hunting with Sam and Dean when they first hit the road again. You and Dean kept that sibling like bond... while you and Sam turned to a more... sexual and romantic relationship.

Neither of you told Dean, though. He'd flip a lid. His little brother sleeping with the girl he saw as his little sister? No good.

But... part of Sam knew he was falling for you. Even if he didn't want to admit it...

One day, when Dean was out, you and Sam began to get... frisky with one another. You thought Dean would be gone sooner... but he couldn't have walked in at a worse time... while you and Sam were both naked, you on top, having sex...