One of my hardest jobs as the mother of special needs kid is separating normal sibling rivalry (and revelry) from autism-related behavior. It's a real challenge not to "hover" over your kid in general, and especially a challenge when your kid has socialization issues. I try to back off as much as possible, but it's tough when he and his sister start butting heads.
I grew up with two older brothers, so believe me when I tell you that I know siblings will fight. It's an iron-clad fact. They'll also annoy, irritate, bedevil and frustrate each other. On purpose. They'll find all manner of ways to torture each other, from the creative (ask me about the entire evening I spent in a laundry hamper when I was eight) to the borderline abusive (have you ever taken a Barbie doll to the face?) and it's just par for the course.
Anna and David mix it up just like any other set of siblings (especially siblings that are only a few years apart in age), and it's a real challenge letting them do so and knowing when to step in because it's crossed over into an autism-related event.
For starters, David's repetitive behaviors can sometimes lead him to be a bit more than annoying. We tolerate this for the most part (and Anna's usually pretty good this way) but on occasion, she's having a bad day and starts mocking him, or getting in his personal space just to annoy him back and it ends up escalating into a meltdown situation pretty quickly.
And sometimes, David's just having a bad autism day and he's throwing things, or hitting Anna for no reason. This is not tolerated. Just because my kid has autism, it doesn't give him license to be a jerk to anyone. If he can't behave, he gets a time out. If he continues, he's off to his room.
It has to go pretty far to get to that level - I do believe in stepping back and letting them hash out their conflicts as much as possible. This is how we learn to deal with each other, and sometimes, if I don't step out of it, I'll be referee-ing them all night long. They know it's gone too far when Mom says "That's it! I'm done with the both of you. I'm sitting down now and nobody had better call my name unless there's blood on the walls."
I guess part of parenting through autism (and just plain parenting in general) is knowing when to throw up your hands and move into self-preservation mode!