11 year old slapped his sister

[changeoflife]

My 11-year old ds slapped his 8-year old sister round the face this evening. She had a red mark on her cheek. I was shocked. I don't hit the kids so they've never seen me do anything like that. I feel like I don't know this child he is becoming since turning 11. He has become sullen, rude and rarely engages with me at all anymore. When he hit her I didn't shout, just took his phone from him and sent him to his room. He's been there ever since. I tried talking to him about the fact it's never acceptable to hit someone especially a girl but all I got was grunts and "don't knows". He thinks I hate him and favour his sister which is ridiculous but in his mind is true. I don't want to lose him as we've always been so close but I'm at a loss. His dad and I have been separated for years but we are amicable so the children have not seen anything other than us getting on. We certainly never fight or argue.
What should I do? How long should I confiscate his phone for?! I thought the toddler years were hard but this is much worse!!

Goodness, I have little helpful to add but I see your post has not been answered and I can give you a little bump.

How are things now? Maybe now things have calmed down you can talk to him about how unacceptable that was but also reassure him that you love him and his sister the same and let him get any worries off his chest. Could he get back his phone on Friday if his behaviour has been exemplary until then.

Obviously violence against a sibling is completely out of order (and slapping his small sister’s face seems particularly cruel)
but it’s also so common, I remember boiling with hate and resentment at my little brother (similar age gap) and we would fight with unleashed fury and I have never fought physically with anyone else my whole life.

Good luck, I hope this is an isolated incident, plus can you get his dad on board? You shouldn’t have to shoulder it alone?