My 10yo DS is a lovely boy generally, but he's reached that stage where we're starting to get a bit of backchat, and some mean teasing of his DB (8). A typical example: both DC's at the table. DS1 will say something a bit mean ("DB can't do 7 times table, what a baby!"). DS2 will start to get upset (he's a drama llama anyway). I'll tell DS1 to stop, but he'll carry on quietly, and then just laugh to himself, that cheeky laughter when they know they've done something wrong and think it's hilarious.
It's at that point i see red. I just can't take the bloody insolence. So I'll send him to his room, or tell him no console at the weekend, or some such. Then he'll get really upset, and I'll get accused of treating him unfairly.
And on it goes 
But I'm not sure if I should just actually ignore it! I'm pretty sure this kind of behaviour is not untypical for a child of this age. And teasing sibling did also normal. That I really struggle with as I'm an only, so have no reference point for sibling relationships. DH, however, said his siblings were all generally awful to each other when they were growing up (they all get on brilliantly now).
How does everyone else manage this? Help!