An in-situ investigation from high-resolution neutron powder diffraction data on the structural evolution of TmNiO3 and YbNiO3 perovskites across the metal-insulator transition, with TMI = 596 K and 598 K, respectively, allowed the charge disproportionation effect that these perovskites experience upon electronic localization below TMI to be followed. In the insulating (semiconducting) regime, the perovskites are monoclinic, space group P21/n, containing two inequivalent Ni1(3+σ)+ and Ni2(3‑σ)+ cations; above TMI, the samples become orthorhombic, space group Pbnm, with a single site for Ni3+. The 57Fe Mössbauer spectra of iron-doped (at 1.5%) RNiO3 (R = Tm, Yb) samples recorded below TMI exhibit for Fe1 and Fe2 (replacing Ni1 and Ni2 sites) hyperfine parameters corresponding to large (Ni1O6) and small (Ni2O6) octahedra. The remarkable difference between the quadrupole splittings (Δ1 ≈ 0.3 mm/s and Δ2 ≈ 0.07 mm/s) of Fe1 and Fe2 sites in RNi0.985Fe0.015O3 is analyzed. We calculate the lattice contribution to the electric field gradient (EFG) at 57Fe ions, and estimate, by using the experimental Δ1 and Δ2 values, the contributions of the 3d, 3p, and 2p electrons (overlap distortion and covalence effects). Above TMI, a unique state for iron atoms is observed, upon metallization of the sample.