To think Dsis is overeacting after SS hit niece?

No.
This kid knows the boy has issues, so despite being told to stop, she carry's on, and on, and on.
Deliberately causing trouble.

If my daughter was involved in this scenario, and I came back to be told that she’s been hit by a boy 5 years older than her, discussing her behaviour would be the last thing on my mind.

If I’d come back and been told of her behaviour, and that behaviour hadn’t resulted in a boy being violent with her, then it would be discussed, she’d be pulled up on not only being annoying and name calling but also ignoring her aunt, and apologies would be made.

however, the SS blew all of that out of the water when he got physical. His violence overshadows it.

I acknowledge I was mistaken about the arcade machine. It does nonetheless demonstrate how quickly and easily angered he is, and his behavior days later shows that he is perfectly capable of resorting to violence against a younger child. He has violent tendencies, and ones he’s fully capable of expressing towards children years younger than him.

OP sat back and allowed the situation to escalate as it did, so no, I wouldn’t particularly trust her judgement on what he’s capable of, or to not to put others at risk.

Legally? He struck a relative in a domestic setting...what legal add ons do you need for your definition?

Legally it wasn’t but I won’t bore you with the law.

I would not ever give my daughter the message that 'she deserved it' due to her behaviour.

You think girls don't need to be parented because women are less likely to be violent than men?

This was domestic violence

OP does say he got angry "at" the claw machine

The niece's words won't ever kill anyone.

Violence in males tends to escalate.

Nowhere in the OP does it say the boy responded violently towards the arcade machine. It simply says he got angry and went for a walk to calm down.

The OP has described anger issues and said that the school is supportive. In my experience if he was violent that support would have gone the way of spit in the rain in a very short space of time.

I have worked with families that care for children and young people with violent behaviours and they are very aware of the risks. I’m sure that if her step son had ‘violent tendencies’ she would have been unlikely to casually offer to look after her niece whilst also caring for her step son and other children including a baby in a confined space.

I am assuming you have little experience of traumatised children and I think that’s probably for the best as you are obviously not well equipped to be anyone’s ‘saviour’. Fortunately there are people who are, otherwise there would be more abused children, more young people in the criminal justice system, more drug and alcohol addiction, more homelessness and more domestic violence.

A child asked to look at a picture in a annoying way, reached to see, water was spilled. Their relative gets up to go ..she is rude to them...says cry baby...then she is hit ...she has been taught that men are volatile and unsafe. I can assure you she has had her lesson about violence and speaking.

If she is aware of this argument in the family she will learn there are 'good' and 'bad' victims of violence..there are excuses for male anger always

DN is learning all the time, DSD is being excused

What would happen to him? That would for social services to decide. He would be away from your vulnerable children, who, like it or not, are a risk from a 15 year old with anger issues that has shown he is perfectly capable of lashing out at younger children.

Arguably, what may happen to them should concern you more than what may happen to him. Like I said, I do hope your sister follows through and contacts the authorities better equipped to deal with him.

I have a question though.

What would have happened with the 10 years olds behaviour if he had been allowed to just go off and remove himself from her and not hit her. Maybe he went for a walk and away instead.

Would your sister and his step dad been supportive if this or would they have blamed him for causing a drama.

Would she have been suitably disciplined for her behaviours.

I strongly suspect it’ll would have been him that’a that is seen as the issue regardless.

Might be worth looking at functional family therapy to see if the dynamics need unpicking.

OP I do think it’s wonderful your SS has got you and DP in his corner and wish him all the best in the future.

But you must see that he has to understand how unacceptable it is to a hit a child? He is 15 - what happens if he gets into a romantic situation with a girl and a girl upsets him and he thumps her?

Your niece sounds like she was behaving awfully btw, and not excusing that. But it is an absolute red line that he’s crossed which you don’t seem to accept.

OP’s sister making a comment to OP about her SS when he had been taken to cool off traumatised him?

Sorry but it’s still a choice to hit someone.

Many many people have some type of trauma. Be that physical abuse, sexual abuse or verbal.

They don’t all go on to repeat that abuse to other people it’s still a choice ultimately.

So how should we raise girls to behave?

What are the reasons that are acceptable to you for an almost adult hitting a junior school child please? It sounds like you think it's justified in some circumstances.
You should not be raising a child like this teenager. You must teach him there's no excuse for it. I can see it now at school, teachers calling you up about his behaviour and it's forever excused. It might not be his fault that he experienced something difficult but it is his and your responsibility to prevent the cycle from continuing. So far you have failed. Please just learn from this and do things differently.

As soon as you raise your hand to assault, its game over.
I dont care if she was dancing a jig and annoying him all week.
He hit her.
Hes in the wrong. Id go to town on him and you. And it would be a cold day in hell before he got an apology..

This is ridiculous when all evidence is that women are taught to fawn to men...there is absolutely no equivalence here ...despite your reaching for it

Actions of a child traumatised by the actions of grown woman.

We don't ask the victims of violence to apologise to their attackers...it's that simple. Once you've crossed the line into violence..we are into different territory.

In this case DSD needs to know he was utterly wrong with no excuses, no prevaricaton.

Yes, they were both rude and should both be punished for that and made to apologise. We need to raise our sons to respect others but we should also be raising out daughters to respect others. Posters mention how will the son turn out but the same can be said for the niece in this situation. If she is mean at 10 and her mother allows her to act that way, her behaviour is likely to escalate in secondary. I wouldn't let my son or daughter behave how either child behaved in this situation and I wouldn't ignore either behaviour.

Point still stands. You're not comparing like for like.