  We study many-body resonances in the neuron-rich $^8$He and the mirror
proton-rich $^8$C using the $^4$He+$N$+$N$+$N$+$N$ five-body model with the
isospin $T=2$ system. Resonances are described with the complex energy
eigenvalues as the Gamow states using the complex scaling method. In $^8$He, we
obtain five states, in which four states are resonances, and in $^8$C all five
states are resonances.We discuss the isospin-symmetry breaking dynamically
induced by the Coulomb interaction in the energy spectra and decay widths of
the resonances in two mirror nuclei. We predict the resonance energies and
decay widths for the future experiments. We also investigate the configurations
of valence nucleons above $^4$He in two nuclei with the $jj$ coupling scheme
and all the states dominantly have the $p$-shell configurations. From the
configuration mixing, $^8$He and $^8$C give the similar results, which
indicates the good symmetry in two nuclei.
