I know it sounds horrid but your daughter sounds like she crying out for attention and feels left out. Especially if there is a new baby! I have been in a similar situation as you and come out the other side. Give your daughter quality time with you on your own and also get her dad to have the same. Because in her world you are mum and dad and she wants it all back to normal with no step parents etc.
Try a reward chart and praise her for the good behaviour, but also ignore the tantrums.
Give it time and it will get better believe me