I have to second Escapefrom's advice, I think it's absolutely spot on - particularly "don't expect any compliance or obedience, sense or understanding, then you won't be disappointed!"
Nearly all stressful times like this can be turned around IMHO, by three things; changing your expectations, distraction, and consistent boundaries.
If she runs off in shops you will just have to be like a brick wall; either she goes in the buggy or she has reins on. Once you've acheived her sat down or reined in, then you unleash a stream of consciousness type conversation and just keep her engaged, interested, in ANYTHING. You both need to immediately move on.
Distraction is still a huge tool with kids this age. I didn't use the naughty step myself, I relied on lots of firm "NO WE DON'T BITE" or whatever it was, then removing my attention and getting on with something else that he could get interested in. I didn't treat this stuff as needing punishment. If ds had a huge tantrum or was being deliberately pesky despite ALL efforts to distract, then I would on the odd occasion do time out, not as a punishment but as a genuine break for us both.
And the main thing as escapefrom hit on, is lowering your expectations. She's still a very young toddler, only two, and she's had a new sibling to cope with. Two year olds are often so unreasonable! But really, trust her that this is her age - she will be your lovely little girl.
Also, does your DP/DH ever give you a break? No wonder you feel you can't cope, with 3, and one of them keeping you up all night. I do feel for you.