  Interpreting the recently discovered narrow exotic baryons as pentaquark
states, we discuss, along an old argument of ours, the isospin mixing occurring
within the two doublets of $Q = -1$ and Q=0 states lying inside the $S=-2$
($\Xi$-cascade) sector. We argue that, at least within the Jaffe-Wilczek
assignment, presently available data already indicate that mixing should occur
at an observable level in both charge sectors, with mixing angles that can be
predicted in terms of ratios of observable mass splittings.
