I think this sounds really difficult and I’m sorry you’re going through it. When I was teaching, sometimes changing the language I used helped.
This formula tended to work like magic: “[Name], I want you to [specific action], is that ok?” Getting the agreement with ‘is that ok?’ makes it very difficult for the child not to follow the instruction. Your instruction has to include something you want them to do, rather than just a ‘stop that’, as just asking them to stop something doesn’t give them an opportunity to take positive action. So, it could be something like, “Amy, I want you to put your phone in your bag and give me your full attention, is that ok?” Be really, really specific - “put your phone down” or “put your phone away” isn’t specific enough, I would say exactly where to put it and exactly what I wanted them to do next. So in your circumstance it could be, “[Your son’s name], I want you to stop pinching your brother and come and stand next to me, is that ok?”
I found that 90% of the time, this phrase nipped any bad behaviour in the bud. But sometimes a child will try and deflect by e.g. pointing out that others are misbehaving, saying ‘I don’t want to!’ Or something similar. Don’t get drawn in, just agree with them & be a stuck record. “Maybe your brother is pinching you too, and I’ll deal with that in a moment. But now, [repeat what you said before verbatim], I want you to stop pinching your brother and stand next to me, is that ok?”
If the child now does what you want, praise them and move on, talk about something else.
If behaviour that you don’t want continues, next phrase: “[Name], if you choose to continue [pinching your brother/ lying down, or whatever], I will have to [a consequence that you definitely can and are willing to enforce, ideally immediately]. Do you understand?” You say ‘if you choose to’ to remind the child that their behaviour is their responsibility.
If they do continue with the actions you don’t want, then: [Name], you haven’t [stopped pinching your brother or whatever it is], therefore you have given me no choice but to [consequence that you said].”
Then the HARDEST BIT - you MUST, 100%, follow through with the consequence.
Follow these phrases exactly, word for word, and hopefully you will be amazed at their power!
I really hope this helps you OP. It isn’t easy to keep repeating like a stuck record and it takes some time to get used to these specific phrases. All I can say is they have worked wonders for me! All based on Bill Rogers’ work who wrote a lot about behaviour management. Good luck - you are doing your best and it’s so hard at times!