You're on the wrong track. These terms are specific to perl's internal representation of compiled perl code. Here, children and siblings are separete things - siblings is a ordered, linked list of nodes that are children to a parent node. The linked list is constituted by a op_sibling pointer, the parent points to the first sibling with op_first, the last sibling with op_last and none of the middle ones. Sibling nodes do not point at any other sibling nodes except with the previously described op_sibling pointer.

This is all about perl guts.