My 9 year old is acting badly about becoming a big sister?

We have a 9 year old & an 8 year old who are immensely close, i'm almost 36 weeks and over the last 4/5 days Brooke (9) has been really horrible to her younger sister, they normally fight but not like this, Brooke's been literally attacking her sister striking out for no reason, and shes been ignoring me and on/off with her dad. Apparently she wasn't very nice towards her best friend at school, and she was sent to the headteachers corridor (punishment) for mis behaving in cl which never happens. She seems v. excited about becoming a big sister as she can't remember Paige as a baby. We told them at about 14weeks so they've known for a while they're going to be big sisters, how do we deal with her? Or do you reckon its her hormones & starting puberty as Paige is completely fine?