Sorry to hear you are having such a tough time.
Just wanted to say that, as you have said, "to be fair to him his behaviour is probably normal", you might find it useful to read some of the intro to Tanya Byron's 'Your Child, Your Way' book. It has some useful comments about how we, as parents, often perceive behaviour of children and how it is possible to "re-frame" our view of it in a more positive light.
If that sounds like too much to do at the moment, it might help you to take a few moments at the end of each day to reflect on when your DS misbehaved, what triggered it, what he did, how you reacted etc., and see if there are certain times of day, or situations, that trigger the TTs or whatever he does. It might be that when he is hungry, or overtired, that he plays up. Also, if you say you are in the kitchen with the baby, avoiding being with him, might it be that he is playing up in order to get your attention? If you think this might be the case, and it is a vicious circle, perhaps you could ask your sister to watch your baby for an hour whilst you do something with your DS that you can enjoy together. Even a trip on the bus can be fun for a 2 year old, so it needn't be anything fancy.
I do hope you get through this. Being a mum looks like such an easy option until you have kids and realise the enormous amount of effort, time and patience required. Everyone struggles from time to time, if they don't they're kidding themselves. It will get easier, but do ask for help from your DH, sister, GP, friends if you need it - even just venting to someone can clear your head and make you feel better, at least for a while. And don't feel guilty about all of this, you are doing your best for your kids