The interpersonal relationship between two individuals bound by such a close social bond that they might be called "family" is featured.

Notes:
Any defined character relationship can be familial if it is clearly considered as such in a story and if it is sufficiently relevant to the plot. It may be noted that while there are many stories in which parental, sibling, half- sibling, step-parent, sibling-in-law, or even adoptive relationships are important, there are few stories in which consanguinity relations beyond first cousinage is relevant to the plot.

Examples:
"guardian and child" is a "human familial relationship" because the guardian is typically expected to take on the role of a parent even if they are not related to the child by blood.

References:
https://en.wikipedia.org/wiki/Family