Answer
sometimes
Work Step by Step
If $A\subseteq B$, then every member of A is also a member of B.
If $B\subseteq A$, then every member of B is also a member of A.
If $A\subseteq B$ and B contains members that A does not, then $B\nsubseteq A$.
Two sets can be subsets of each other only when they contain exactly the same members.
