Castiel was there the moment Dean woke up from being freed from hell and they have been fighting along side each other since. Also breaking the rest of the seals took much longer and Castiel became close to the brothers while they helped him defend the other seals from breaking and releasing Lucifer. So the group has been together for a while now.
Dean and Sam were in a hotel going through adds and obituaries looking for their next case when Castiel appeared in the center of the room looking panicked. The angel immediately started pacing the room back and forth so quickly that his movements caused the papers the boys had scattered about the table to fly off into the floor.
“Cass what’s wrong? You’re freaking us out over here.” Dean spoke loudly to grab the angel’s attention.
Castiel stopped mid stride and turned around facing the humans he considered his comrades, his friends. He walked up to their table and stopped. He didn’t know how to explain everything to them. He didn’t see this coming. Nobody could have predicted this change events.
“We………” Castiel fumbled with the right words. “have a problem.”
Dean lifted one of his eyebrow’s while Sam snorted.
“We always have a problem, what is it this time? Has another seal been compromised?” Asked Sam.
“I believe you may find this worse..” Castiel sighed running his hand through his hair.
“Worse?” Dean asked surprised. “What’s worse than the seals being broken and releasing Lucifer?”
Castiel looked Dean in the eyes. He would just have to come out with it. He just hoped they wouldn’t be too angry with him for keeping her existence a secret.
“Your sister is in hell….”
Dean and Sam looked at each other than back at the angel before they both began laughing. Castiel sighed irritated. “This is NOT FUNNY!”
“We don’t have a sister Cass.” Dean chuckled.
“Yes you do…. You just didn’t know about her. Your father never told you he had another child.”
Both Winchesters immediately quieted their laughing. Their face’s taking on serious expressions. Dean’s worried. Sam’s pissed.
“How do we get her out of Hell?!” Dean shouted jumping to his feet. He had been to hell already, if they truly had a sister he wasn’t going to leave her in that place!
“Who is she? Where has she been? Why didn’t he tell us? How old is she?” Sam fired off every question that came to his mind. He didn’t understand. How could his father not tell them something this important? When it came to John Winchester there were always secrets but this is something they should have known about. This was family!
“Her name is Bella Swan, your father left her with one of his old hunting buddies named Charlie Swan in Washington. She just turned 18 a week ago. She was a premature birth, very small. Your father didn’t want her in this life so he kept her secret. His youngest child a little girl. He didn’t want her to fight for her life after he almost lost her when she was born early.” Castiel than turned to Dean. “I can go to hell and retrieve her like I did you Dean but I will need your help. Heaven won’t be of assistance with this.”
Dean slammed his fist down on the table. “WHY THE FUCK NOT? WHY WAS I SO SPECIAL BUT YET MY SISTER ISN’T?”
“You were born to hunt Dean, plus you and your brother were the ones that started the apocalypse. You’re the only ones that can end it but your sister wasn’t raised the way you were. Heaven doesn’t find her….. useful in this situation.”
Dean flipped the table over shouting profanities. Even Sam became outraged and he was usually the calmer of the two.
Sam shook his head. “She’s out sister. Our YOUNGER sister. I can understand now why dad didn’t say anything especially after some of the shit we have seen and been through… but heaven? This is ridiculous! She isn’t injured or something like that, no. She’s in HELL!”
“Exactly.” Dean said grinding his teeth together. He was pissed off at heaven. Not at Castiel though, he knew that the angel was most likely not supposed to tell them anything. In fact he was going against the others in offering to help them save her. Dean took a deep breath than walked over to Castiel and placed a hand on his shoulder with a gentle squeeze. “What do you need us to do Cass?”
“I have a plan but I need you both to watch my back while I rescue her, cause a distraction to with enough demons that they focus on you while I slip into hell to retrieve her.”
Sam and Dean both had determination in their eyes. They had lost too much already. Nothing was going to stop them from getting back the only family they had left beside each other. This was their baby sister after all.
“Let’s do it Cass.” The brothers chorused.