Well guess what, the guy writing the Boruto novel also wrote the Gaara novel and in it he established that Suna elects Kages based on the Kazekage bloodline. I didn't make it up. He did. So if the Sand Sibs are foster kids like we found out with Shinki, that means that Shikadai is the only one running around with that bloodline and would be Gaara's heir. That was the whole point of the novel. That if Gaara and Kankuro didn't have kids that Temari's Konoha born children would be in line to be Kazekage.
Shinki IS adopted since he called Gaara his foster father. So he is not his biological child. It is pretty clear in black and white. I reckon it is the same for the others. It is not shocking that Gaara wouldn't have kids of his own. He didn't care if Temari's children only had the Kazekage bloodline and were from Konoha. And it would be in his character to adopt children of his own and raise them as his own. Sort of like Jiraiya kind of did.
There was a character in the Gaara novel named Kajura that had the Iron Sand kekkei genkai and he was from a village between Suna and Iwa. So the notion that Shinki can only be from Gaara, Temari, or Kankuro due to a bloodline is already out the window.