i dont think this is a spoiler because its pointed out in the anime.
Hanabi is now the hier to the Hyuga name due to the fact that Hinata was such a failure, they just tossed Hinata aside and now she isnt worthy to a true memeber of the main family.
This is why Hanabi is still in the main family whilst Hinata is too


Hiashi is the member of the main family because he was born first whilst Nejis father is a member of the branch family because he was born second.
This is when the seperation started, not before then

Therefore any child of Hiashi's will be a main family child and any child of Neji's father or his will be a member of the branch family. Its that simple