Some one has uploaded the D23 Expo for The Lion Guard on Youtube
Ford Riley (Who has worked on Disney Junior shows in the past like Special Agent Oso) talks partly about how Kion is the second born cub of Simba and Nala, and that his place in The Circle of Life is different than his older sister Kiara, who will lead the Pride Lands.
When he says "the younger brother of Kiara, the future queen of the Pride Lands.", you can hear some one gasp, a gasp that seems excited for the return of Kiara, who has only recently been seen in current Lion King re tellings, in this form:
It may not be as big as a role as Kion has, but Kiara being future queen is important to the show and helps define Kion and his place in the Circle of Life.