I am a 17 year old boy so this isn't from a mother's point of view.
I have two sisters. My 15 year old sister has been ridiculously ill-behaved since I can remember. So bad, that I and most people would consider it abuse. She has assaulted each of us more times than I can remember, some being as serious as the time she repeatedly hit my sisters head into the corner of a wooden cabinet resulting in a permanent scar and an a&e trip. My mum and sister both admit to me regularly that they are scared of her, and when I'm not there the fear increases a lot. This is a huge issue because I stay 40 minutes away from the house 3 days a week for my college course. My sister is abusive at least twice or three times a day - on a good day. We as a family do not feel safe with her around.
My mum's boyfriend is a brilliant guy and makes my mum's life bearable. I stay at his house a few days a week because he lives near my college. He is also my employer and has essentially replaced my dad; I wouldn't be happy if my mum was with anyone else. My sister has degraded and is still ruining there relationship, because they agree to refuse to move in together due to my sister's poor behaviour as he has kids of his own. I feel as if if they move in together, they will split up. They have been together for 11 years.
It's been a problem for ages, however my mum hasn't involved me in assisting her since I turned around 15 (involved me as in started talking to me about her problems with her) which i think is because of the fact I've always been considered mature for my age and because my dad isn't in the scene.
My mum does absolutely everything for my sister. She buys her whatever she wants, she pays £40 a month for a phone contract, music school and literally every thing a teenage girl could want. She has social workers, people who help her with mental health and family support (which ended as I was fed up with being dragged into meetings trying to fix the family when actually the issue resides within her). We live in a rural area as well, so the majority of these meeting are very far away. These people have been involved in my sister's life since she was 7 (I think).
She abuses my dog in multiple ways such as hitting him, picking him up by the collar and more.
My mum has moved my sister schools twice upon request and discussion (due to bullying) and is now home schooled. She doesn't have any friends because she craves attention and drama, and this isn't coming from me. Not only has this been diagnosed on multiple occasions, but she's also admitted it more than once. She's also been diagnosed as a psychopath.
I'd say that her behaviour negatively affected all of my school life, results and definitely my 12 year old sister's.
Every single person who has witnessed her disgusting behaviour (family, friends, the street, local police officers, councillors and members of the public such as the local shop staff who had to clean up after she smashed up a child's scooter she stole last week) has agreed that her moving out is a good idea.
I believe my mum has considered putting her into care, however the social worker (I think - there's so many people involved I don't know who controls what anymore) hasn't allowed this to happen. My dad refuses to have her as well. I feel as if this is very wrong.
I'm really getting to the end of my tether with this. No parent should become depressed due to a child (my mum is now on anti-depressants due to her behaviour). I've seen my mum break down crying more times than I'd consider it fair. If anyone has any suggestions please let me know, any input would be appreciated!
Many thanks, and apologies for the really long-winded answer.