He does sound very 'normal' though. Tell a teenager no and Jesus the wrath!
It can be hard to recognise them at this age, hormones are playing havoc, their moods are up and down like yoyos, they snap back, they shout, they get cross, they push boundaries and suddenly they're a foot taller than they were in summer.
I'd say the key is consistency, realistic expectations, A LOT of positive stroking and kindness. Also the recognition you are uncool.
Everything else will muddle into place.
The PC is a great example. He knows he can't go on the computer after nine during the week, that's the usual rule. You expect him to come off at 9pm but if he's finishing homework/nearing the end of a game perhaps he can have an extra ten minutes a couple of times a week. If he breaks those rule then comes the discipline - if it works he loses access to his tech for misbehaving.
When he's off his tech try and spend time with him, see if he'll help cook tea, go on a walk, day out, help with homework, watch something on tv etc. Praise his participation in these activities and he'll find that there's other stuff he enjoys other than PC.
If he tries to wind the sister up then it's further removal of tech after a warning or two. He is old enough to realise the consequence of his own behaviour. He's twelve and picking on his sister because he's bored/feeling hard done to, not two and having a tantrum he's at a perfectly competent age to realise the long term implications.
If he loses the computer for a week or more then so beit. It's a consequence of his own actions and he's choosing to behave like that.