Talamare, Hiashi and Hizashi were both sons of the previous head family member, right? But Hizashi, being the younger one, became the a BRANCH. Now let's forget about that.
Hinata and Hanabi are both daughters of the current head family member, right? Now wouldn't the same thing apply to Hizashi too since he and Hanabi were born second?
I don't think Hyuuga Hiashi could just change his mind about it. The laws and ways of the Hyuuga are absolute aren't they? Why else would Neji be so angry with them at first?
Feanor has a good point on the Shikamaru thing. That could be it.