The structure of isomers in N ∼82 nuclei is reinvestigated. It is pointed out that the quasi-spin structure survives in the isomers and the final states of their decays, as apartial dynamical symmetry Using this group structure, an extended seniority reduction formula is derived and the presence of pair excitations out of the Z=64 core is revealed. By taking into account the core excitation, an anomaly in neutron effective charges for N=81 and N=83 nuclei is greatly reduced.