Sounds like jealousy to me. I think all three year olds have tantrums, my GS does, although as he is approaching 4 they are getting less and less.he seem to dig himself a hole and not know how to climb out, so to speak. Think seriously about Drs. And councilling, it might just feed into her about getting special attention. Obviously she needs to be punished ( stand in the corner until stop crying etc) and try not to make a big deal of it, hard I know. Never ever compare her to her younger sibling, give her a few minor responsibilities for the baby's care, and take her home with you for a night or two. Then when you are doing granma's things, making cakes whatever, together and she is feeling like your world, have a chat. 7 year old level. Not about how wonderful the baby is, but how tired mummy gets when she acts up, and how mummy loves her forever and needs her. Tell her she can come or call you if she feels funny and doesn't know why. Mine ask their mummy if they can call me, although we are lucky to live around the corner. Good luck, patience is needed. Tell daughter to read Dr. Spock