The generalized Ward identity is derived for the vertex function of the weak vector current (VC) of nucleons within an effective theory with account of the isotopic symmetry breaking. The generalized Ward identity establishes a relationship between the form factor of the longitudinal weak VC component and the mass difference and the charge radii of an isotopic doublet on the mass shell. This relationship can be used to evaluate the form factors of both nucleons and nuclei with spin J = 1/2 and isospin I = 1/2. The reactions with large transferred momenta involving isotopic doublets, in which the mass splitting is maximized, offer the greatest promise for measuring the form factor. Numerical estimates of the form factor of the longitudinal weak VC component of a nucleon pair and eight nuclear isodoublets are presented.