Don't step in but back mum up by saying 'do as your mum tells you' (and I would be quite harsh about it, Liz is obviously nicer than me ) without coaxing or attention, and then leaving mum to dish out the consequences. You have to give your daughter the chance to see that mum's in charge too, atm she knows she can act up for quite some time before you come to rescue her (mum) and any child would naturally revel in it. If you feel you can't resist the temptation to step in, go for a walk round the block to take yourself out of the situation which will also let your daughter see your just not playing that game anymore.
I imagine your stepping in knocks mums confidence a fair bit too so you'll be helping her too.