What would you do in this situation? PLEASE HELP?

In my opinion. A swimsuit is to be worn not put in a picture frame. It was a present from your mother to be worn and soon she will have grown out of it and it will be of no use anyway. You should have let her wear it and perhaps she wouldnt have got angry. She on the other hand is a handful and is disobedient and in my opinion with a mother like hers (not you) she isnt helping and is happily goading her daughter to mess you around. Not good and isnt fair on you. I think you could however be making yourself out to be the big bad strict stepmum when really you could chill a little. Dont strip her bedroom - she will hate you and run from you and make things a whole lot worse. Kill her with kindness, be kind (even if it is thru gritted teeth) be loving, and ignore her childish ways. But just dont let her get to you AT ALL. Just keep smiling and When she sees she cant get to you cos you dont care shel soon get bored and will have no reason to play you up and will hopefully grow up and see how much petty times shes wasting.