Try lots of cuddles and praise for her being a helpful girl, or eating tidily, or using lovely manners, or going to the toilet without having to be asked, or listening well, or telling great stories, or for being your favourite 3 year old. Whatever praisworthy thng you can catch them doing.
The "favourite x year old" works really well when there's absolutely nothing you can come up with that they've done that day worthy of praise. You know, those says when they trash their room, spill their water, drop food all over the floor, pull their sister's hair, (perhaps poke or otherwise wake the babies in your case) and so on.
IMO, 3 is an age where they are learning big concepts: independence; that mummy and daddy can't always know what they are thinking/feeling; that they have an opinion which may not be shared by siblings or parents; they may be going to kinder or childcare and learning things from ppl other than parents.
A bit of reassurance can go a long way.
Then, consequences. As in, she hits her sister, she loses the right to a shopping treat. Be as consistent as you can, as that adds to the reassurance that all is well in the family and the knowledge that new babies (or anything else) are a normal part of her life.
Good luck; I can't guarantee that these will work (although the favourite x year old has worked on several small children of my acquaintance), but as you can see from my sig, I have just been through two of these! They were so very different in how they behaved, you wouldn't credit it, so I can't presume to advise on particular ways of dealing with certain. behaviours.
You will get through it!
Edited by poss71, 22 December 2012 - 07:17 AM.