To think Dsis is overeacting after SS hit niece?

I agree. But I am the parent of a boy. Primary school age girls were a bloody nightmare. They are often much sharper witted with language so can argue the toss about anything. At my son's primary school I think most of the mums thought their DD's shat diamonds. I was the strictest mum in the playground. I did get apologies over time, as people realised they had perhaps got lucky with child 1, but child 2 was very different.
@opi think your SS still needs help. It is lovely that his stepdad and you care so much to support him. My son, when younger, would also revert to lashing out; there were a lot of problems in the family at the time and he literally had no other way to express it, ie the trauma. He is now a young adult and doing amazingly, but i do still wonder what the long-term consequences will be. .

I was taller than my mother when I was 10 and a little bit taller than the 14/15 yr old boy who hit me because I tried to phone for help when he assaulted a boy who had (according to my attacker) been assaulting him. (Both boys came running into my room. The one who was the 'victim' and hit me was at the back.)

My being taller than a 15 yr old male made not one whit of difference. A teenage boy tends to be stronger than even a grown woman.

As a teacher, I sometimes had to break up fights/stop assaults (usually in a playground or corridor). By the time boys are 14, it's often impossible for a woman to restrain them.

So what should I have done, told my son that he should apologise but so should she?

No way. His behaviour was inexcusable. As was OP's DSS.

Younger than 15. Maybe 12 or 13.

Horrifying the words they pick up. He's not heard that in my household.

I'd hope they'd default to that but if they were upset and wanted to talk then I'd be happy to try and explain.

I haven't said she deserved it at all. I have said her behaviour shouldn't be ignored or justified, in the same way the girl in your sons example shouldn't be.

I agree. It's a shame that the question needs to be asked at all. As parents we should surely all agree that both children acted in a way that isn't age appropriate and both children need support to change their behaviour.

You keep saying he’s getting help for ‘anger issues’ but you are failing to realise that in the meantime other people and especially children are exposed to these anger issues. Your sister, rightly, is trying to protect her own child and even without the actual assault of her daughter, she has witnessed, or knows about your SS’s violent temper. This is making her uncomfortable. Put yourself in her shoes and try to be more understanding.
At the very least, your SS needs to be very closely monitored by your husband for the rest of the holiday, but in reality this type of holiday might not be best going forward until his issues are more under control.
It’s not fair on him or the others whose holiday he is making uncomfortable.

Her behaviour clearly wasn't great but it doesn't mean she deserved to be hit by a boy 5 years older than her!

His behaviour should not be minimised because she was being provocative!

He was violent towards her. She didn't 'deserve it'.

The OP shouldn't be expecting an apology and saying she was 'just as bad'. She should say that it is never OK to be violent.

I feel sorry for her DSS as he is not being taught what is and what is not appropriate behaviour because OP is minimising his behaviour.

My son once called a girl a cunt when she was being rude to him. I was told by the leader of the drama group that they both went to. He said to me that she was being awful to him. He has NEVER done that again. I didn't care what she said to him, she didn't 'deserve it'. I made him phone her mother to apologise for his behaviour. I didn't expect her to apologise that her daughter had goaded him.

He is now a very well balanced, respectful of women , adult.

I'm sort of struggling to understand why you couldn't stop the 10 year old, OP. Generally with children of that age, if you speak in a strong enough tone that. makes it very clear they have no choice but to obey, they do it.

A 15 yr old boy hitting a 10yr old girl is never ok. Your sister is not over reacting.
You watched it all happening, but chose not to intervene. If the boy has issues then it's on you to step in before he reacts if you're responsible for him whilst around your young niece.

So what happens when some kid at school/wherever acts like a dick (as some nasty kids unfortunately do!) and says something really triggering; perhaps about his mum as unfortunately that’s a common insult etc.
If he can’t stop himself being wound up by a 10 year old girl being a brat, without resorting to wacking her, what would he do when really really emotionally triggered?

OP, are you at that point still going to stand there making excuses for his reactive behaviour? Because the police won’t buy into that and he will find himself in a lot of trouble.

Now is the time to tackle this before it gets worse, yes therapy is great but your reaction too holds tremendous weight.
stop making excuses, yes he has had a shit time, it doesn’t make his reactions ok.
Yes, she was being a brat, he tried to walk away etc, but bottom line is- she is a much younger child.
It’s great that you love him so much; and you sound extremely supportive but sometimes that clouds judgment.
Step up, discipline him in some way instead of essentially permissively bleating ‘it’s her fault she made him do it’ etc etc. It’s not sending him the right message at all.

It's doesn't sound like he is getting any support her original post states they're "just patient with him". Patience isn't going to improve anything it's just showing him that it's ok for him to be angry and lash out because they're going to wait for him to grow out of it

Oh, add psycho to the list of names

Well, seeing as you seem to think that the poor little 15 year old boy needs protecting from the annoying, big, bad 10 year old girl, you’ll have to forgive me for not taking your word on the possible reactions of your children

I can’t believe that some people have reduced this post to ‘Do you think it’s okay to be emotionally abusive?’ Lol.

Admittedly mine are younger. At 15 I like to think I'd be able to explain that she's a little psycho who just wants a reaction so be the bigger person and ignore her.