Screaming at the children

I've always babysat my sister's kids and to be honest, I'm in doubt as to whether or not I'm cut out for this, or ever even have been. My temper just boils over to the point where I just want to murder them and hope I get away with it.

I have two nieces and a nephew; 4, 5 and 7, respectively. The following are just a few examples of the shit I have to put up with.

Fairly recently the youngest sneaked into the kitchen and got into the chocolate powder; she knocked it all over the place and even tracked it around the tile with her bare feet. When questioned about this she lied and said it was her brother. I then reminded her that that was really quite impossible, as she is the only one who could have left footprints that small in the kitchen. She then lied again and said it was her imaginary friend Anastasia. That pissed me off even more and I called her a liar and a dumb-ass, among other things.

More recently I was laying on the couch with my nephew and watching the old Ghostbusters movie. Out of nowhere the little bastard pinches my fat and says "Haha, Aunt Trish! They should have cast you as the Marshmallow Man!" To put it simply, I lost my marbles and slapped the shit out of him.

The middle child, while generally less of a pain than her siblings, is not exempt from obnoxious behavior. It was just two nights ago when it came time to put the kids to bed and she came up with the bright idea of sleeping in the backyard on the trampoline. I tried explaining to her that as a lone little girl she would be completely at the mercy of pedophiles or werewolves, but she refused to heed my reasoning. What ensued was a screaming match so loud it very well may have attracted the attention of alien lifeforms from distant galaxies. Anyway, it ended with me body-slamming her into bed and demanding that she stay there or that I would see to it that she turn up missing. She refused to call it quits, and in response I held her down by her wrists and screamed mockery in her face until she got the message. This took no less than 13 minutes.

What do you think? Do I need help or do my sister's kids just need to stop being assholes?

Is it normal to get this damned angry at them?