I only own this idea. Enjoy! :)
Thirteen years ago a tragedy happened, according to the two rivaling clans. A Uchiha gave birth to twins. The father is a Hyuga. The parents were killed by their clans. The head of the clans were forced to take in the children. The Uchihas' got the boy and the Hyuga's got the girl. Everyone was forced to keep it a secret. But, how could a boy ten years old keep it quiet that they killed his mother? He was forced under adoption.
The twins eventually grew. The girl was forced to wear contacts. She was never allowed to show her true eyes. When the children turned five, they found each other. The girl was running away from her 'father' because he was beating her. She ran all the way to the Uchia compound. She ran into one of the buildings and hid in one of the supply closets.
Some people were walking past her. One of them stopped. "What is it, Itachi," another one asked.
"Somebody's here." Itachi answered. He stepped in front of the closet and slammed the door open. The little girl looked up and started backing away. Some of the supplies fell on her. "Come here," he commanded holding out his hand. "I promise I won't hurt you." The girl looked at his hand and grabbed it.
Itachi picked up the girl and headed to a room. He then set her on his desk. "I have to go," the man that talked to Itachi earlier. "When your done with her, tell me if your suspicion was right." The man with blue skin left.
"What's your name?" Itachi asked.
"Hinata," the girl answered.
Itachi smiled. "That's what my little sister's name was."
Hinata looked at him confused. "Was?"
"She was taken away from us Uchihas. She has a twin. My little brother, Sasuke. When the two were born Hinata was taken away from us."
"Why?" Hinata cried. "Why did they do that?"
"She's a Hyuga. She looks exactly like you."
Hinata realized what he was talking about. "Am I your sister?" Itachi smiled and nodded his head. Hinata also smiled and hugged him. A loud knock was heard.
"Quick hide underneath the desk!" Hinata scurried under the desk. As soon as she was under she saw a boy about her age hiding there also. He put a finger up to his lips and she nodded her head. "I'm your twin brother, Sasuke," the boy whispered. The door then slammed open. Hinata rushed to her twin and hid her head in his chest. Sasuke wrapped her arm around her in protection. "You are not allowed to be here, Hiashi."
"I saw my daughter come in this building. I thought she would be in here."
"She's not and she is NOT your daughter."
"At least let me check."
"No. I have been here all day no one has come in here."
Hiashi growled and slammed the door. After three minutes, Itachi gave the all clear. Neither one of the children came out. He looked underneath his desk to see Hinata sleeping on the boy. Itachi laughed at the scene and his brother's face turned bright red. "Sasuke meet your younger twin sister, Hinata."