I've been seeing a really great therapist for the past year, and she mentioned that, in every single one of our sessions, I mention my sister as the biggest source of stress in my life; from what I tell her, she says that it sounds like my sister has BPD. I researched online and I came across this sub. It really does sound like she's borderline. 

I'll try to keep it brief: she's my twin sister, and since she was a baby, she's had issues with being very needy, jealous, and aggressive, so my dad's solution was to spank both of us for fighting, and my mother's solution was to spoil her to keep her quiet. We were mostly raised by nannies because our mother (is still) married to her work, and because parenting has never really been our parents' thing.

They divorced when we were 8 (the second divorce for my dad, third for my mother—they have a lot of issues of their own and their generation's attitude to mental health and personal problems is to bury them and pretend they don't exist) and it really fucked us up, but especially my twin. She got into violent media and depression porn type stuff as a way to cope, and her childhood issues spiralled into worse anger management issues and explosiveness. 

My mother comes from money, so finances were never an issue, but my mother basically decided that the best way to try and contain my sister's problems was with ADD meds and appeasing her. I was, and still am, extremely resentful of how my mother finances my sister's every whim; though we still live with her and she pays for my schooling, any additional expenses I ask for are denied and I get lectured, while my sister cycles through a ton of pets that she later ignores for the maids and me to care for as well as money for weed, outings with her "friends," alcohol, clothes, etc. I get a ten minute speech about how an echocardiogram for my elderly dog is too expensive, but will fork over her own gas money to my sister without a second thought. 

Because my mother prefers to bury her head in the sand whenever a problem that isn't work related pops up, it's created worse problems for all of us. A chaffeur raped my twin sister when she was little (and an older sister) as well as spent years siphoning money and stealing from the family. A cook who hated us and her colleagues and the pets and did things like spit in the food helped him to do this. My mother knew all of these things. She did nothing. They were only recently fired this year because they both poisoned one of my sister's latest dogs and my sister had a breakdown. 

My sister has been through four relationships in the past three years and they've all ended extremely badly. There's always mutual mistreatment, featuring fighting, going through each other's phones and social media, and mutual infidelity. My sister would cry to us about how she's been kicked and raped by her boyfriends, yet would also shove and scream at her boyfriends in front of us. Her relationships are always extreme rollercoasters. 

She also engages in risky habits like substance abuse (weed, alcohol, she confessed to having tried cocaine at some point), casual sex with dangerous people, going out at night to very seedy areas, and her "friends" are people who hang around her because they know she'll buy them food and drugs and such. She's attempted suicide several times before in the past, has engaged in self harm and has eating disorders, and has even downed a handful of pills she hoarded instead of taking as scheduled to coerce my mother or make her feel bad. My sister has jumped around several therapists and psychiatrists before and she says all of them have mistreated her. I don't know how much of that is true given that she hates to talk about her problems in-depth. 

My sister is incredibly sweet and funny and kind when she's not off on a screaming tirade about how the world is against her and how we all hate her, and then immediately swinging into a sobbing mess about how she's all alone and afraid of being alone. She's a grown woman so I can't force her to get the help she desperately needs. I just wish she'd see past the fog and get herself professional help, and for my mother to take her head out of the sand and stop denying what's right in front of her. 

I'm tired of having to listen to nightly screaming matches and to wake up to a house that pretends that nothing happened. I'm tired of having to tiptoe around my sister to avoid setting her off. I'm tired of being the bad guy for trying to enforce boundaries. I'm tired of having a mother with a spine as strong as a wet noodle. 

But in the end, I know I'll be fine, once I get a job and make enough money to buy a place of my own. But I can't say the same for my sister. 