Example:Family relationships are relations

From CS2800 wiki

There are many ways that people can be related to each other; and these are all relations. For example, "is-a-sister-of" is a binary relation on the set of people; [math](a, b) \href{/cs2800/wiki/index.php/%E2%88%88}{∈} is-a-sister-of [/math] if [math]a [/math] is a sister of [math]b [/math].

Similarly, you could define an "is-a-friend-of" relation, or a "loves" relation, etc.