Abstract: Isospin symmetry breaking is essential to exploring atomic nuclear properties, offering vital insights that further our understanding of nuclear structure, reactions, and astrophysics. Mirror energy differences, a primary observational quantity in the study of isospin symmetry breaking, allow for a clear reflection of this phenomenon, which is significantly meaningful in comprehending the properties of nuclear forces. In light of the remarkable progress in supercomputing capabilities and quantum many-body techniques in atomic nuclei, ab initio calculations have achieved great success in nuclear systems. In this study, using the chiral effective field theory two-body and three-body forces, we employed the ab initio valence-space in-medium similarity renormalization group method to calculate the low-energy excited states of the mirror nuclei ²⁵Si and ²⁵Na. The nuclear forces adopted consider both charge symmetry breaking and charge independence breaking effects well, and the Coulomb force is also included in the quantum many-body Hamiltonian. The results show that the three-body forces are crucial in describing the excited states of nuclei. Based on the results of the excited states, we calculated the mirror energy differences and corresponding occupations of states in mirror nuclei. The results suggest that the larger mirror energy differences are primarily caused by occupying the weakly bound 1s_1/2 orbit in proton-rich nuclei.