  We use vector flavor symmetry to relate form factors of isospin changing
operators to isovector form factors. Flavor twisted boundary conditions in
lattice QCD thus allow isovector form factors of twist-two operators, e.g, to
be computed at continuous values of the momentum transfer. These twisted
boundary conditions, moreover, are implemented only in the valence sector.
Effects of the finite volume must be addressed to extract isovector moments and
radii at zero lattice momentum. As an example, we use chiral perturbation
theory to assess the volume effects in extracting the isovector magnetic moment
of the nucleon from simulations with twisted boundary conditions.
