My daughter has a twin. Her twin sister passed away when she was 12 days old and it’s been 3 years now. I noticed my daughter talking to the wall and saying hi girl and saying love you to this girl. And then I was going through pictures today and she looked at her picture and said other sister is the girl she seeing. Could it be her twin sister?

If she's talking to a wall end saying "hi girl and love you"
Then she might seeing a ghost, show your picture to her and ask her are you seeing her.
Spirits are real
Yea I know this sounds very strange but you just have to believe her what she's saying after you ask that question.
Hope it goes well.

How often do you talk or tell her about her twin ?

Countless children have imaginary friends, so it's nothing to worry about (not that you are), and I agree that you probably have talked about her twin often enough that she has created her. But there are many mysterious things about twins that even the best science hasn't explained yet, so I would nurture this without making a 'ghost' out of her.
Serious studies of twins show that even when raised apart from birth, some share very specific individual traits. And that some really do 'feel' what just happened to the other twin, even if thousands of miles away. Not always of course; some twins don't even get along.

Countless children have imaginary friends, so it's nothing to worry about (not that you are), and I agree that you probably have talked about her twin often enough that she has created her. But there are many mysterious things about twins that even the best science hasn't explained yet, so I would nurture this without making a 'ghost' out of her.
Serious studies of twins show that even when raised apart from birth, some share very specific individual traits. And that some really do 'feel' what just happened to the other twin, even if thousands of miles away. Not always of course; some twins don't even get along.

I didn't know that the scientific people that studies about twins. :O