Oh, where to start? I have the world's most immature sister, and she is an annoying piece of stuff. She fails at everything but annoying people. People at her school hate her, and I don't blame them. She steals things, slaps random people, vandalizes other people's property, and gets horrible grades. The surprising thing is, my parents are biased and take sides. My parents divorced when I was little, and every other weekend my sister and I live with my dad. I live mostly with my mom, who seems to like my sister more than me. My sister does everything she can to piss me off, including stealing my money and personal possessions, destroying my property, and playing loud music and singing along in her dying donkey voice. Whenever I politely ask her to stop, she will not stop, and does the act that I tried to prevent more frequently. I can not do anything to get revenge, as my mom will beat the stuff out of me with words and absurd punishments. My sister could do anything short of blow up a bus of orphans and nuns and not get in trouble. Whenever I go to my mom to get her to intervene when all else has failed, she just gets pissed off and yells at me and I get grounded, when I never did anything. My dad on the other hand, seems to favor me because I am better behaved than the monstrosity that (impossibly) has the same genes as me. My dad is not afraid to discipline both me and my sister, the latter happening more frequently. He knows whenever my sister steals something or antagonizes, and she gets punished quite nicely for her bad deeds. Then, my sister will go to my mom and say that my dad slapped her, and then a three way stufffight will occur with me and my dad versus my mom. The reason these problems happen is that my mom refuses to help me solve them and then stops all progress that anybody is making to discipline my sister. What do I do to keep my sanity?
tl;dr My sister is an idiot and my mom does nothing to stop her reign of terror