The nuclei, i.e., , and , establish an interesting isospin isobaric system. and are unbound broad resonances, whereas is deeply bound in its ground state but unbound in all its excited states. The present situation is that experiments so far have not given consistent data on the resonances. Few-body calculations have well studied the scatterings of the systems. In the present work, we provide many-body calculations of the broad resonance structures, in an ab initio framework with modern realistic interactions. It occurs that, indeed, , and excited are broad resonances, which is in accordance with experimental observations. The calculations also show that the first excited state almost degenerates with the ground state in the pair of mirror isobars of and , which may suggest that the experimental data on energy and width are the mixture of the ground state and the first excited state. The isospin triplet formed with an excited state of and ground states of and is studied, focusing on the effect of isospin symmetry breaking.