My brothers are being obnoxious and don't do anything?

alright so im 15 and i watch my twin brothers that are 11 every day when my parents are working. One of them (david) behaves great! he acts fine and just does his own thing until the other one (daniel) starts being obnoxious-and im talking to the point when hes running around singing songs he makes up about people and screaming/making noises for no apparent reason. All they do is play wii and ps3 ALL day and we have a pool and a trampoline but they refuse to go outside. They leave wrappers and food all over the place, don't flush the toilet, pee on the toilet seat and don't clean it up, wrestle constantly, an eat like everything we have in the house. Its ridiculous and when I politely ask them to pick it up they like scream in my face "NO BURT" (burt is what they call me for some reason even though my name is Kate... and then god forbid i have a yogurt or something because they like scream that im fat(even though I have worked wicked hard and have lost 30lbs) and make up fricking songs about it. they are also like mean to my grandparents and do things just to agitate them which is dumb. I'm sick of it and if i have to beer with it much longer im going to snap! so i was wondering if this was just a stage or if there is anything i can do? my mom knows but doesn't do anything at all...thanks!:)