"you're my mum, you should do stuff for me, if you want me to do stuff back that's selfish"
Not witty, but the kind of reply I find effective
'I'm your parent which means I have the pleasure of guiding you to becoming an independant, happy and productive adult. Your the child and your role is to make stupid statements so I don't have to be so bloody reasonable, you were saying?'
The second part would be spoken in a slightly sinister manner. I rarely swear and when I use a 'bad' word it is a good indication she is getting close to crossing the line.
"what perfect world are you living in if you expect me adn (brother) to be kind?"
This is easy -
'The same perfect world where you expect...'
Then add in something they want to do and require permission, something they want/need you to do for the them etc. I find at this age that tends to be a great deal. Then top it off with
'so what do you think, will we both be dissapointed?'
Being prepared to follow through if they call your bluff helps.