Texinfo refers to (non-root) top-level sections as “chapters”. In line with Notenik’s concept of Web books, I’m quite fond of this term myself. In our case, this is useful to solve the complexity of how to refer to the siblings of a child note’s parent[1].
A merge variable like =$next-chapter$=
could suffice. It may be advantageous to round out the suite of navigation variables and compliment =$next-slug$=
by adding =$prev-chapter$=
and =$prev-slug$=
as well.