  The concept of a relatively weakly injective pair of operator systems is
introduced and studied in this paper, motivated by relative weak injectivity in
the C*-algebra category. E. Kirchberg \cite{Kr} proved that the C*-algebra
$C^*(\mathbb{f}_\infty)$ of the free group $\mathbb{f}_\infty$ on countably
many generators characterizes relative weak injectivity for pairs of
C*-algebras by means of the maximal tensor product. One of the main results of
this paper shows that $C^*(\mathbb{f}_\infty)$ also characterises relative weak
injectivity in the operator system category. A key tool is the theory of
operator system tensor products \cite{KP1,KP2}.
