it sounds like the girl's saddened by the breakup of her own parents. this is normal. she felt safe/comfortable where she was. she had no say over what happened. now u, the step mom r in her life. this is probably very hard 4 her 2 accept. u can never replace her own mom, but u can if she lets u, b her friend & help her thru this.
i'd look @ the possibility that maybe she 'feels left out thus isolates herself' as mentioned b4. i strongly feel that this very same thing may also b the case in many adults' lives. those w/ few contacts; who r wrongly judged by others. when rejection results, adults can feel left out & isolate themselves, 2. i sure hope researchers read this & do a serious study on this issue