That's because of divison of an existing power which isn't the case with Borat's and Himawari's dojutsu. There isn't anything already existing for their to be divided.Not necessarily. Genetics can also be very random and luck based even amongst siblings and other family members. Even in the NV:
Asura didn't get sharingan or rinnegan did he? Yet Indra got sharingan.
Hamura didn't get rinnegan.
Hagoromo didn't get tenseigan or byakugan.
Flawed logic the genes exist and passed on whether they manifest or not. They may manifest within a later generation the fact is the powers still have an original established source. For Borat and Himawari neither parent possess the dojutsu to pass on or divide it between the siblings so Himawari would inherit it also if it was possible.No known descendants or siblings of Hashirama got mokuton (unless Moegi is secretly his descendant, but that still doesn't take away the fact that Tsunade didn't get it, nor did Tobirama, and probably not Itama, Kawarama, Butsuma, or Nawaki either)
Boruto didn't get byakugan.
Temari and Kankurou (and also Shikadai) didn't get magnet release from Rasa (yet Gaara apparently did).
Etc...
Existing traits I agree with, just as all Uchiha carried the Sharingan gene yet not all manifested it. They still possessed the ability to possess on the unmanifested Sharingan genes to the next gene. There was no random dojutsu passed on by them via ass pull for the sake of plot. I hope you see the flaw in your comparison between the two.Do you see what I mean? When it comes to genetics, even if a child has the potential to inherit a trait (based on their parents having the trait), that doesn't guarantee that the child will get the trait. This also means that just because one sibling gets a trait, doesn't mean that the other automatically has to.