To be honest I think your step daughter is feeling left out and maybe jealous, because her father lives with 2 more children, in her eyes she feels her father has replaced her.
Does her father have quality time alone with his daughter?
Also what do your daughters think of there step sister?
I know this is harsh, but my 3 daughters have been through the feelings of jealousy, angry, left out etc!!
But deep down all they wanted was to be loved and feel secure have lots of quality time.