Gauging the Horseplay or Countdown to the Screams

We were at my sister's for the holiday. This morning, we were watching their 2 dogs who were playing. They were wrestling around a bit, so I told them to stop. Sylvia told me, "but they're just playing." I told her, "yes, but that's how you and your sister start. And before you know it, the physical play starts getting too physical and someone gets hurt."
The dogs managed to hold it together, but I stand by the remark for the girls.
It's that blasted balancing thing again - balancing the "just having fun" to the screams and the tears (and possible bruises).
I keep hoping that they're getting old enough to outgrow this stage. But I think I remember my nephews still doing that up to last year! Does the fact that they're girls give me any hope at all?
I'm sure that some of it is from just being together too much. They're only apart when they're at school (and then they act like they haven't seen each other in years), and they do have to share a bedroom, but at the same time, they refuse to spend time in separate rooms unless and until I demand it.
I can relate to a certain extent, but at the same time, my sister and I always had our own rooms. Then, she was 6 years older than me, so she stopped living with us when I was in junior high.
When I was in 7th grade, a teacher asked me if I were an only child. I said, "yes, but I have a sister." The answer made perfect sense to me!
I honestly don't remember us getting physical with each other, although I wouldn't be surprised if my sis or parents remembered it differently.
I try, of course, to use it as an opportunity to tell them that there are better ways of dealing with their anger, but I also understand that feeling of sheer rage, where reason is the furthest thing from my mind. So I know I can't get through to them at that point.
The only thing that I've found works is just trying to separate them as fast as possible. Or, if we're in a moving vehicle, blasting the music so loud (and belting as loud as I can on top of it) that the audio overload will momentarily make them forget why they're so mad at each other.
No matter how many times we go through this, though, they still look at me so innocently when I tell them to watch their horseplay...no sense of the foreboding that I see no matter how many times we live the experience. When will the recall set in?!?