The other day my younger sister and I were in our room. So she kept calling me a lesbian. I keep saying no so then she says bisexual person and I said nothing. She said she didn't care but I didn't want her to find out now. I'm still happy she accepts me but she doesn't understand why I am keeping it a secret from our dad's side of our family.
I assume your sister is old enough to understand things when you explain them to her? Tell her why you keep it a secret (I don't know your reasons). Make sure she understands that everyone takes their own time coming out and it's something you have to tell them when you feel ready. Maybe she doesn't understand it because she doesn't care, so tell her that not everyone will react that way and coming out is really hard and difficult.
My brother is 11 and he has no idea what being gay means. I think my mom told him once that some guys like guys and he was like wuttt.