My STB stepdaughter is giving me the silent treatment AGAIN..?

She doesn't like change. She probably feels like you are trying to step into the role played by her mother. You should have girl talk, with just you two and tell her that you are not going to take over that role, she doesn't have to call you mom, and that nothing will really change. Just relax. She's a teenager. Teenagers are bratty. Just talk to her one on one. If it doesn't work then a family meeting with her dad might help.