hold up a minute.
She's not necessarily toxic, though if she meant to hurt you that was, yes, a toxic thing to do. You come from a toxic family.
You say you 'bounce back' more quickly from life's vicissitudes - well, you would. She was brought up in the siberian wastlelands while you were all warm and toasty. You'll have a LOT of reserves to draw on.
You say she's 'bitten the hand that feeds her, emotionally' - I wonder how she feels about that (genuine q)? ie that you, the princess, are always there to mop her - the broken, hopeless one - up. I'm not saying you're not lovely to do that, but I doubt she particularly enjoys that dynamic. Does she have the opportunity to mop you up, too, or are you generally sorted and don't need it?
Talking talks huge sense here, especially the cognitive dissonance (which looks like willful ignoring to me). I am both the family's golden child and the scapegoat, depending who is involved (though overriding scapegoat tbh). I know all about the siberian wastes but also about being jabbed at for being the favourite - not minor jabs but major, longstanding, neverending, pure poison, on and on.
To say 'comparison is the thief of joy' is not factoring in natural, automatic sibling comparisons. It will be torture for her to have you in her life, I imagine. Absolute torture. She who can do no wrong/get nothing wrong/has it all spangling away at her side. I feel for her. No wonder she wanted to pop your bubble. To answer my own 'genuine q' ^^ I think she has made it clear how she feels about it by writing that letter. (not that I've read it OR condone it).
Get real OP. It is not an equal relationship and you are kidding yourself if you think it is. yy it's lovely to get on with ones siblings - I wouldn't know - but how can you not know how your childhoods have impacted on her on the deepest level? You don't know because your life hasn't hurt you enough to do some digging and find out. Hers has.
Her one outlet - one she probably got a big hit from having published and receiving support - is now brought to ashes, she is pilloried for being disgusting. I agree it was a horrid thing to do and would have been better bashed out in private with a therapist because, hey, relationships are complex. She actually has been a victim btw and that takes a great deal to come back from. If one ever does tbf.
You've managed to have a fairly functioning relationship so far and, I have to say, hats off to her in large part for that. I hope you can see that.