dss suddenly walked over and slapped her hard on the arm, for no reason
Did he tell you there was no reason for slapping her? Just wondering, if you actually asked him.
Also, the very things that make 7 year olds funny and loveable and witty (i.e. their over-the top emotions and impulsiveness) are also things that can make them do things that aren't so nice, and even mean to others.l But he's 7. And being 7 means he will do the wrong thing.
Whilst, wrong, I don't think your DSs behaviour is unusual. What I do think is unusual is your reaction:
I lost it and shouted at him and told Dh I wanted him to take him out for a few hours before I said something bad to him. Really don't wasnt dss in the house tonight.
Your house is his home. He is 7. And you can reach a level of rejection of him that would lead you to say you don't want him in his home tonight.
I couldn't look to him right now
I won't have my baby hit in the Place she should be safe, not by anyone.
And how about him? He's 7 years old. He lives with his Dad and not his Mum full time, which makes one imagine there is some level of a back-story. There is now a new baby who belongs fully to the two parents in his home, but he doesn't. You've had PND. It seems like there's a lot going on for this little boy. Maybe he had no reason he could articulate, but maybe he has a reason, or may he doesn't understand, and just lashed out. But maybe rather than being shouted at by you, and his Dad, made leave the house, be treated quite nastily before he left (the idea of not being able to look at a small child and having him taken out of the house and not wanting him back that night, is a very harsh response). Imagine how he must feel?
I also think you need to gain some perspective on what having a sibling is like. They can be the best of friends and the worst of enemies all in the space of 5 minutes. They can be scratching each others eyes out one day, and the next defending the other against all any sundry in the school yard. She is a baby. He made a mistake. He needs to learn from that mistake. But more importantly, you need to learn how to have proportional responses to quite usual behaviour, and not deal so harshly with such a small child.