My little sister is tearing our family apart. Im lost with what to do?

Im 17 and shes 14. Undoubtedly, she has always been the centre of attention and I dealt with that. She has always played up to get what she wants, these last few years it has gotten worse and worse and this last month nearly every day there has been a screaming match beteween her and my mother. Its been over getting this new puppy of which we cant afford. My mum told her time and time again no. If she (my sister) feel thats shes has had any little injustice she goes crazy throwing things. Im just as angry with my mother because she dosent punish her, she even said to me today "What am I supposed to do?" Im so sick of this that I burst into tears today. I understand that teenagers have spouts of misbehaviour but this isnt ordinary. Im ready to tear my hair out.