Why do they do this?

My sister and my grandmother are tearing my family apart piece by piece. My sister isn't the smartest page in the book and has done some stupid things. Some of the things I really hate her for. She used to go out with her friends and smoke, drink and she had underage sex and when my parents found out they were mad, and my sister threatened to leave and they were like "Go on then you have nowhere to go" and she said she would go to my grandmothers house. My grandmother said that if she left she could go to her house, because she thinks my parents are being unreasonable about what she has done! Unreasonable?! They have every right to be mad if she has been doing those things. Plus the fact that my grandmother favours my sister much more e.g. she bought my sisters prom dress, shoes, tiara, jewellry and paid to have her hair done. She didnt get me or my little brother anything. My family have noticed that she has favourites and it isnt right because everyone should be treated equally. My sister has been going to a pub with my grandmother to play darts and has been coming home drunk and my grandmother doesnt see anything wrong with a drunk 16 year old. My family is always fighting with each other because of her and she doesnt give a damn. I hate her and just want her out of my life, because she and my grandmother are slowly but surely separating my family.

well i think you shud sit down and talk to your paretns and let them know that they are ripping you apeart and im sure they will see the better side of you and not your sister! i haven't expierenced this so i dunno but i think u shud talk about it!
luv dre x

Write them all long letters, to your parents, grandmotehr and sister. To each of them explain your fear of you family breaking apart and tell them how they are helping it happen. Then with that mind set get them all together to talk, on your terms.

I agree with that. I think that you should get your family together to talk or go and talk to your sister yourself. You never know, it might help.