step children

hiya, i live with my partner and his children, as well as my bd. (biological daughter), seems lately there is a problem with the older 2. his daughter and my bd.  problems in association with his copying my child, getting the same clothes as her, doing the same thing, saying the same things, buying exactly what she has, socks, undies, bags, pencil cases etc.  too name a few.  my bd id finding it increasingly difficult and is getting frustrated, but reacts either in an assertive manner or will just do her own thing, but each results in the other child crying.  it is driving me nuts.  what can i do.  loosing sleep at night. these kids are all under the age of 11.