PROLOG
"Who shall bring this child down to Earth?" asked an angel during a joining.
"I will." said another.
"Very well, may she be with you through all trials." replied the first angel.
"Be sure to give her the most beautiful vessel, Castiel." joked the first angel.
Castiel nodded in agreement and set off for Earth.
"This vessel should do." Castiel said aloud, looking at the dead pregnant woman on the ceiling.
"The Winchesters should treat you right. I'm sorry for putting you in an unborn child's body." continued Castiel.
You both appeared in a car. Both of you sitting beside each other in the back seat.
"Hello Dean." Castiel greeted the man driving.
"Holy sh- Cas! I thought I told you no appearing when I'm driving!" Dean shouted, looking at the sleeping man beside him in the passenger's seat.
"Sorry, but I have a gift for you." Castiel said with no expression on his face.
"Please don't be another 'Castiel's Homemade Pies'." Dean whispered to himself.
"I have your sister." Castiel continued, picking up my hand and waving it at Dean for me.
"Cas, we don't have a sister." Dean replied, grabbing the bridge of his nose.
"Just take the poor girl back home to her family." Dean continued.
Castiel looked at me confused, and said to Dean.
"Is this not the child your mother was pregnant with before she died? The unborn Winchester?"
With this, Dean looked in the mirror at me. He swerved the car over to the side of the road and parked.
"Everyone out! Including you Sam!" Dean yelled, punching Sam in the shoulder.
"Hey! Wh-" Sam started, looking down at the twelve year old that was myself.
"Who's she?" Sam finished, stepping out of the now known '1967 Chevy Impala' or 'Baby'.
"As of now, our naked 'unborn sister'." Dean answered, sarcastically.
Sam looked at me again. Then at Castiel.
"Our mom never mentioned her being pregnant before she died." Sam said, confused.
"Because she didn't even know." I explained.
"I am your sister. I am twelve years old. I was a twelve year old angel and took your mom's dead baby as a vessel." I shrugged.
"And you need some clothes, here." Dean said, giving me his jacket. Then he slapped me across the face.
"Ow! What was that for?" I complained, slapping him back.
"If you were a demon it wouldn't have hurt." Dean explained, placing his hand on his face.
"Well obviously I'm not!" I shouted, also rubbing my face.
"I would've killed you first, duh!" I continued.
I started to feel something running from my eyes.
"OH MY GOD! I-I'M BLEEDING FROM MY EYES! CASTIEL HELP!" I screamed, running to Castiel.
"No you're not! You're crying. Relax." Sam comforted, sitting me down in front of him on the grass.
"Here." he said, giving me a white piece of cloth.
"This is a tissue." he explained, "It helps dry your face when you cry."
He stood up and started discussing something with Castiel.
"So, 'sis' what's your real name?" Dean asked, sitting beside me.
"Alecia." I replied, looking down at my bare feet.
"Let's make that name shorter and easier to remember. How about Alec?" Dean suggested, helping me up.
I shrugged.
"It seems okay." I replied, stuffing the tissue in the jacket's pocket.
Dean half-smiled and walked over to Sam and Castiel.
I stood there speechless, both worried and excited for my new life with the Winchesters and Castiel.
A/N
So this was a short chapter for the beginning of my new Supernatural Fanfic, but, we'll call it the 'prolog'.
SEEMS LEGIT.