Stepmother problems?

Give her time and space. She will come around. Be there when she needs you, but don't force her. Let her father do the parenting for now but support him. Even though you are not trying to replace her mother, she may not see it that way. She's young and needs time to accept the changes in her life. Support her, love her, and be her friend. That's about all you can do for now. Good luck.