Fab I totally understand what you say that no punishment works on them as I am having this problem with DS(9.9yrs) and have been for the past 5-6years.
We tried the naughty step/chair/corner, grounding, removing of privileges, no pocket money and canceling school trips and after school activity's, he doesn't respond to any of them and his reply is always "I don't care"
He lies, runs away from home and school, steals from us, is rude in school to the teachers, is aggressive with friends and his sisters (who are 3 and 4), breaks things, hitting and kicking us and the most distressing at the moment is that he actually keeps accusing us of hurting him.
If he hits/kicks out at us we will try and restrain him and if it's just me or DP dealing with it on our own he will scream for the other saying that we're hurting him (for example DS went to punch DP in the face but DP caught his arm and said "no you don't hit anyone" let go of his arm then tried to ask him why he was so angry but DS started screaming "mum he's hurting me dad's trying to break my arm" I know he was lying as he has done the same to me, he said i was strangling him when i was on the other side of the bedroom on a chair and he was on his bed.
We've tried doing more with him but he then starts acting up halfway through the activity or says he doesn't want to do anything with us and would rather go out or play on his own, so either way we cant win.
The biggest problem is that his sisters are seeing him getting attention from acting naughty so are now behaving the same as they think that will get them attention as well and that's probably why your DS2 is now behaving like it too.
I cant give advice as I'm battling this too but wanted you know that it's not your fault, you haven't done anything wrong, it is hard and there are people who sympathize with you and know what your going through.
If you want to chat let me know and I'll send you my email address, if you don't that's ok too