You're not alone. Your sister sounds just like mine. Myself and per siblings are either incommunicado with her or just indifferent.
I don't think she realises how daft she looks or how much she is actually disliked.
She will make Facebook posts like, 'I'm the biggest bitch you'll ever meet, but I'm worth it'
and her huns will like the post; 'Oh Hun, you're so brave and forthright and honest'.
It's more than just drama queen attention seeking. Someone explained to me that everything they're accusing you of, they're actually guilty of or lacking in themselves, which causes their insecurity.
For instance, like my violent ex (who's Facebook posts she likes to copy onto her own Wall, because she admires him or something we must assume
) she will say things to me like, 'Everybody sees through your butter wouldn't melt act, you can't stay so nice forever, you'll get caught out eventually, you big fake'. My ex said excatly the same words.
It was a lightbulb moment, it made me re-read all the accusations my ex had ever levelled at me, and saw through them all for the insecurities they really are, about him, not me.
I've become indifferent to her since social media appeared. It's made me see a side to her I knew existed, but also made me feel sorry for her.
It's a handy format for people like that, bullies. They publish as much vitriol as they can make up, and you're defenceless to it, you can't reply.
But she's aware of my indifference now so her rage has no audience. No point bullying if the victim just shrugs and walks off. It's how I'm dealing with my DV ex too now. His abuse dissipated significantly since I stopped caring.
OP, don't wait for explanations is my advice. Whatever's driving her isn't something she'll ever make herself vulnerable enough in front of you to share.
My family dismiss it lightheartedly as 'sibling rivalry', and say my sister and I are having 'a feud'. What they're actually doing is condoning the behaviour of a bully, and I'll never get that, the same way I don't get why my violent ex still has friends who have witnessed his abuse, his despicable treatment of me, and continue to remain his friend.
Be assured, everyone knows what your sister is like, she even knows, she knows her Facebook huns are only superficially agreeing with her rants, deep down she even knows that her own family are embarrassed by her behaviour, but she's caught in her own downward spiral forever now.
I feel deep pity for my sister but I can't help her. She has a level of arrogance that's unbreakable. She's too much effort. For anybody. Which is reflected in her broken relationships, career history, general lifestyle failures, family feuds, constant dramas. Nobody wants to make an effort with her.
But her behaviour is toxic and I have children so sadly they lose their relationship with their Aunt because of it.
Sorry to hijack a bit with tales of my own sister, OP. But to summarise: nothing will ever change. They're not wired for it.