I do not believe questions from siblings about behavior or similar topics should be accepted, whether they are written as from siblings or rewritten to be a hypothetical. I fundamentally believe they are not capable of being good questions, unfortunately, and thus should remain off topic. A question that is about something concrete ["How high should we build a shelf to childproof for my sibling", "At what age does a toddler start to walk"] is fine, but not a behavior-type question (i.e., not a question that can be interpreted as "What should be done in [x] situation").
The fundamental problem with these kinds of questions from siblings, and the question that led to Anne posting this shows this clearly, is that they are written with the inherent biases of a sibling. A person cannot have an unbiased or neutral point of view in relation to their sibling; they spent a decade or more with that person, and have built up a bevy of emotional connections and responses that are impossible to remove. The poster will have strong opinions on how the sibling should and should not behave, and strong opinions on how their parents treat the two of them in different ways, that are impossible to remove from the question itself.
These questions are inherently detailed, specific questions about a particular person's behavior or actions. The linked question for example goes through a laundry list of the faults of the sibling, every personality trait that the sibling has that the poster disagrees with. It is not a hypothetical question; it is a specific question about a specific person. What it is not, however, is a parenting question. There is no actionable parenting choice here, because it is not asked by a parent.
A parent will inevitably have choices to make, and these parents could certainly ask a question about this particular child; but they would not ask this question. They would have a different point of view; they undoubtedly are making some effort to raise this child, after all, even if not sufficient in the poster's point of view. But the poster cannot answer the questions we would inevitably ask the parent - what have you tried, how does the child react to that, etc. - in part because they can't see the efforts the parents are making (as they are not always there, and were gone for years), and in part because of their inherent bias.
The poster ultimately isn't asking a parenting question, in any event; they're asking a personal relationship question disguised as a parenting question. If they did get a good answer to their question - imagine a simple, actionable answer was possible, even though it rarely to never is - as to what to do with their sibling, the answer wouldn't be actionable by the poster. The poster additionally needs to talk to their parents and convince them to follow the instructions, and/or needs to talk to their sibling and convince them to follow the instructions. Those are both key, vital pieces of the answer, which are not part of our scope; they might be on topic at Interpersonal Skills, or on the defunct Relationships site, but not here. That's usually the real question - the 'Y' in the 'X-Y' question - in these questions.
These questions also unfortunately do not add very much value to the site. They don't encourage evidence-based answers, as they attract answers that are fundamentally opinion oriented - at best, experience-based answers, but those ultimately become opinion in most cases due to the differences in the situations and the lack of available detail. The answers tend to be fairly specific to that particular child, and the question itself is inevitably particularly detailed to the point that it makes it hard to see another parent getting value out of it. The particular child at question is almost certainly not going to get any value out of it - not answered as a parenting question, anyway.
As such, I believe that it is incorrect to allow a hypothetical version of these sorts of questions to be on topic (as well as incorrect to allow a non-hypothetical).