My Unrelated Brother

Akira Hisato is my Brother's Name.
We are not related, but we're Family.
Probably you can call it that I was from the "outside" family. .
Our Father is the same but have different Mother.
But since Mom died from illness,
Father and Aunt Miu, Akira's Mother, decided to take me in and I also became a Hisato.
I was previously a Fumihiro, my Mom's maiden name.
She didn't marry Dad so she stayed as a Fumihiro until she Died.

My name is Ayako Hisato.
Akira and I live in the same roof.
He treats me badly.
He always hates me.
He always say that I'm not his Little Sister.
He's so Cold when I'm around. .

But when he met with his friends,
He smiles and always treat them kindly. .
The opposite of what he Treats me at home.
Well, same here in the same University we attend.

He's famous around the Campus.
We are both in out 9th Grade--15 years old.
Our difference in age is only days. .
'Bout 8 days. . I was born in the Month of June and the Date of 25.
He was born in the Month of July 3.
Every single one in the Campus are curious about us.
But he refuses to talk.
Sometimes, he said we aren't related.
Well, actually that's true.

Why is Akira so cold to Ayako ?

Does he hate Ayako that much that he can ignore Ayako's existance ?

What did Ayako do to Akira that causes this situation ??