My sister is a dumb bitch.

My sister is 12, i am 18. Everytime she sees me, she says hi and i dont respond. Then she comes over to me and says hi again and i dont respond. She keeps saying hi repeatedly until i finally say it back. She says ''Oh, only now?'' and i say yes. She asks why i am acting like this and i say i am bored. She asks if i am bored enough to play a game with her and i say ''How is that gonna happen? Is that gonna make my boredom better?'' She gets mad and leaves. She constantly does this and it is very annoying. Can someone please tell me what to say to her?