Being in a situation where i have my eldest children but often these days im lucky to see them once a week, due to their boy issues and him being better equipped to deal with them, i am getting your dd's plight..
My boys are 10 and 9 this yr and they are crazy about their dad and he left us for another woman whom has another child ( not his), my kids always had resentment towards him and always would act out due to the abandonment and other issue's.

I dont mean to sound mean and i dont mean to be patronising but i totally agree with artechim... too her she is seeing her mum build another family with out her in it, and to her it probably seems that this baby is going to have you from day one and she is still waiting till she is old enough to be part of it. this is going to be a very tricky situation and if she is feeling like this its only going to get worse, and alot more harder once this baby comes..
Although she may be hitting puberty, she is going to be riding all the hormones that are going to make her alot more difficult to understand and also its going to affect her alot more due to her going through puberty...