My DD is nearly 2 and has been hitting, throwing and generally upsetting her older brother who is 4 more and more for a few weeks now.
I could see she wanted his attention and i dealt with the behaviour using the naughty step and warnings consistently.
It didn't really work and she would happily taker herself there, say sorry, and hit him again 2 minutes later. DS is very gentle and she hasn't learned it from him, he never hit her until she started it out of attention seeking and it only happens from him occasionally.
It got to the point where i felt the neighbours must think i just tell them off all the time, and DD went off me for a bit because i was doing a lot of disciplining.
I had a long think about it and realised that if she was trying to get DS's attention, maybe she had tried to get mine positively and failed, so was just going for bad attention. (not necessarily your situation but i hadn't really looked at things from DDs point of view. When i did i felt really bad. She can't understand why i'm washing up etc instead of playing.)
Over the last couple of weeks I have made a big effort to praise her when she does things well, like someone else said it's easy to leave them alone when they're not causing trouble and i have definitely been guilty of that (not saying you have at all!) We all clap her if she walks somewhere nicely, helps me or puts her shoes on etc (DS gets the same positive attention). It has made a massive difference. I have continued with the same consitency over naughty step etc but it has been used less often.
I have also found that giving them a clearer routine has helped. Sounds like you already have that and so do we in some ways, but they didn't really know what it was - now we have posters with pictures of what's going to happen in order e.g. bedtimes, and that has helped both of them so much - they are more in control of what they do.
This may or not be relevant to your situation, but after a few weeks of slipping into a situation where i was angry with them fighting and DDs hitting etc, i have found we have been able to come out of it in a positive way and i feel much better.
You have my sympathy, you must be exhausted after dealing with it for so long! It is totally normal!