This is normal behaviour, I had three girls, then I did specialist Foster Care for hard to place cared for children/young people and guess what?, they were nearly ALL girls and teenage girls at that. The best way is to remain as calm as possible, not enter into arguments even better get out of the way as soon as you can and focus on something else, even washing up etc and as Anne said, tell her you are extra sensitive at the moment due to he baby and on that point, no matter what sort of relationship you have or had with any stepchildren, they will have difficult wit whatever you do and are bound to say you are not their mother, even if there mother was the worst person in the world, she is still their mother and often, step children will test to see how far they can go, also to get reassurance, remember, the baby maybe a threat to her as well, a good thing to do is to ask her for advice! ask her about her very early memories and re enforce, it is her flesh and blood as well. A lot or teenagers go off to Uni or similar and when they are there, parents have no (or maybe they do) idea of the things they are getting up to!!!!! I have had the lot from my three, Eating disorders, drug use, mixing with baddies!, violent boyfriends, mood swings and aggression etc etc...,stay calm!!!!!