So i messed up

I have this bad habit of snapping at my sister. And.... being dismissive when she asks me things. Today, she asked me something and I said, I don't feel like answering her and I will tell her when I want to talk about it.
She said I was losing my manners and if I continue this way, I won't have many people left to talk to.
I felt bad.
I talk well to the others but not to her. :(