The relation 'is a sister of' in the set of human beings is____

A relation is said to be transitive when an element $a$ is related to $b$ and element $b$ is related to $c$, then $c$ is also related to $a$

We know that if $a$ is the sister of $b$ and $b$ is sister of $c$, then $c$ is also a sister of $a$

Thus, the relation " is a sister of " is a transitive relation.