Arti, can you give her a job to do with DS? For example, she is always the person who has to get the nappy, hand you the wipes, stick the tapes down and put the nappy sac in the bin. That way she feels important and useful, with lots of connection to the baby. As far as the whining goes I think you just have to ride it out. Her little world has been rocked in the last 2.5 weeks and she will feel that you only have eyes for DS, even though you spend loads of time with her. Can you spend some time with just her, with A elsewhere, so she feels special? It feels so against the grain to go out of your way with them in the face of vile behaviour, but it is easier for you all than the other.
In terms of defiance, I (try to) look disappointed and say "oh, it's a shame that you won't put your shoes on / stay in bed / leave the cat alone as I was hoping we could go to the park / shop / watch a film / do a jigsaw, but that won't work if you can't listen to me and do as I ask". That gets the right response some of the time, which is better than nothing. I also feel as though I am shouting less and feeling the love more for the kids.
DD2 was only 18m when DS was born, so still a baby, but DS now is quite a biddable fellow and I expect your DD2 might respond to this type of treatment. Worth a try? It's awful when they are wilful, though, isn't it, and I have to work hard not to lose my rag and yell at them, which makes us all feel shocking. Hugs to you XXXX