We are no more capable of diagnosing PDA or ODD or ASD than we are of diagnosing a brain tumour or liver disease. Just stop it.
Firstly, talking of things people need to "just stop", I would say comparing autism with brain tumours and liver disease would come high on the list. Difference has benefits as well as deficits.
And while I agree that online diagnoses are destructive, whether that be insisting parents are sociopathic or that kids are neurodiverse, I've not seen anyone do the latter - did I miss it? I've seen people with autistic children suggest it is worth exploring as an option, alongside the OP and her husband seeking help for the family dynamic. It's impossible for anyone here to know what underlies this situation, but given ASD has a strong genetic basis, and the OP has one diagnosed child, and girls are massively under-identified due to different presentation, I don't really understand your issue with posters suggesting she should talk to professionals about the situation? I mean, it doesn't actually matter what is causing this. What matters is that the family, including the DD, have appropriate help and support in moving forward more constructively.
Personally I think she may be impacted by being the sibling of a child on the spectrum. It's not easy, especially if the autistic sibling hasn't had good support, and acts in a very tempestuous and controlling way as a result, and the parents have focused on that child, and lost management of everyone's emotions and the needs of their neurotypical kid. But again, I can't know if that is remotely relevant. It may have no application at all.
What I do know: it is always necessary, when situations reach the point described in the OP, for families to learn ways of managing their relationship with their kids more constructively. Which is harder when they are very explosive - whether due to autism, or unfortunate family dynamics, or just situational stresses. Nobody here has a clue what is causing the problems here. All any of us can know is that professional help is clearly needed.
Not sure how anyone thinks attacking the parents will improve matters, either. If there is a fucked up dynamic and they are the creators and perpetrators of that, as several posters believe, then they still need help if their daughter is to be helped. That's axiomatic.