  An isospin analysis of $B\to \pi\pi$ decays yields $\sin 2\alpha$, where
$\alpha$ is the usual CKM angle $\alpha\equiv {\rm arg} [-V_{td}
V_{tb}^\ast/(V_{ud}V_{ub}^\ast)]$ without hadronic uncertainty if isospin is a
perfect symmetry. Yet isospin symmetry is broken not only by electroweak
effects but also by the $u$ and $d$ quark mass difference --- the latter drives
$\pi^0-\eta,\eta'$ mixing and converts the isospin-perfect triangle relation
between the $B\to \pi\pi$ amplitudes to a quadrilateral. The error incurred in
$\sin 2\alpha$ through the neglect of the resulting isospin-violating effects
can be significant, particularly if $\sin 2\alpha$ is small.
