I find this to be a normal reaction to change. You're in a tough situation- my kids have grown up with other kids in their house from almost day one, so there wasn't much acting up- they think this is normal
I think all you can do is be firm with both kids with your rules. This might seem too easy of an answer, but make sure your daughter gets lots of hugs, kisses, reassurance that she is still your #1. When my boys start acting up, I find it stops when I am extra loving. Adding more kids eventually should help, but our own kids are usually the hardest!