3 teen girls, 2 years between each. Kids fight but our rule is no one ever raises a hand at another person. They can yell and stomp and cry and hole up in their rooms but they can never EVER hit. They have learned other ways to cope with their emotions.

Everyone has their less than stellar parenting moments. Easy solution-keep their nails trimmed. DD can wear makeup on her scars when she's older.

When my kids do this it's a draw. They are both told to knock it off and not get physical with each other. We have a 'Love List' on our fridge. You do something good, you move up. You do something bad, you move down. This includes us parents. (I moved down a lot!) First person to get to 10 on the Love List gets their wish (within reason). They do not like to be moved down.

In a situation like that I would move them both down on the Love List and it would be over.

How I read your post:

My kids were scrambling up the stairs, something happened, and DS hit DD. I exploded at DS, did not attempt to get his side of the story, and, after a bath, threw him in a dark room with no further parental contact for the night including no chance to debrief the incident, acknowledge his feelings, and talk about why what he did was wrong. I was especially upset because my daughter's physical appearance is of primary importance to me and something I fixate on. She is a fragile girl, who I need to protect, and only later did I realize she may have been partially at fault, at which point I yelled at her too (and now I feel bad about that; far worse than I feel about yelling at her younger brother).

Of course, I'm probably reading way too much into your post, but if things transpired anything like the scenario above, I think you could do much better in many areas.

Sure, I've lost it at my kids too, especially when they fight (which is pretty rare). I've sent them to their rooms, and don't give them a chance to justify themselves in the moment during sibling squabbles, but I always go in a little later, give hugs, and talk about what happened and how things could have gone better. If I feel I over-reacted (and pulling a kid into a room forcefully would count) I apologize, but explain why I reacted so strongly. Seeing a parent lose it can be pretty traumatic for a 5 year old. Often if they hold it together or act like they don't care, it's when I come in to cuddle and talk about what happened that they show how upset the whole thing made them. Siblings will fight. The incident you describe is worth punishment, but I don't think you handled it well.