the adjustment period after having a new baby can be tough on little ones. I have to say the last few months have been the hardest with my dd and she's two and a half too. I totally understand the non stop demanding and then meltdowns over the smallest of things.

The things that works best for me is taking time to reconnect with dd and reignite the positive energy. I find the more negative attention she gets the more negative behavior I see. I know it's tough with such a small baby and all you want to do is run away and take the baby, but if you can really try and talk softly to her with cuddles and reassurance you might find an improvement. What about a bath together after baby goes to bed, or a cuddle at bedtime with some stories. Basically something with lots of touching and soft kind words.

As for the discipline, I have heard 1-2-3 magic is good but haven't tried it myself. I find positive reinforcement the best strategy which is basically ignore the negative and praise the positive (and ignoring is really really hard when you just want to explode).

Good luck.