Re: How to discipline a child (becoming difficult)
Congrats Saadia. I remember when you were just a little girl yourself!! Masha-Allah you are now the mother of two!
Along with the great advice that you have read in this thread I would encourage you to allow the "big sis" to participate in some of the care-giving for the newborn.
Noor had a tough time in the beginning when my nephew was born as they were living with us. She didn't act out as much but she did become stubborn and resistant when asked to do something. So her aggression wasn't so much directed at the new baby but it was there in her own actions towards other things.
Let her help you with tasks that she can manage. Let her know that the arrival of the little one has helped to move her up the ranks and that with the added respect she will get she must also take on added responsibility. She may enjoy the idea more than you expect.