Why won't my sister act mature with me?

I'm the older brother of a 12 year-old sister. She's already smart to the point she skipped 2 grades. She's currently in 9th grade. She has ability to carry conversations with older adults and can be sophisticated. Why won't she talk and act like that with me? I asked her an elaborated question (I... show more I'm the older brother of a 12 year-old sister.

She's already smart to the point she skipped 2 grades. She's currently in 9th grade.

She has ability to carry conversations with older adults and can be sophisticated. Why won't she talk and act like that with me? I asked her an elaborated question (I knew she had the answer to) but she replied with an ''I don't know, I'm gonna go the movies with my friends''.

She reverts back to acting like a kid with me.
Update: Another time I tried to give her a sheet full of complicated chemical problems and exercises as well as some of Einstein's work. She played dumb, seemed bored and said ''I don't know that'' and went out skating in the park.
Yet, she shows her capacities with others but not with me.
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