When I was 10 and my brother was going through a "phase" he used to act like this, lasted from when he was 14 to when he was 17. Don't worry your sister will probably grow out of it and look back and think "wow I was an idiot" like my brother does now.
My family doesn't eat together. I mean it's not that we argue, we don't do that anymore, just that money's tight and everybody's got to work a job at some point so we're never home at the same time.