High-resolution x-ray absorption measurements reveal a rare-earth-dependent splitting of the Ni K edge in the insulating, charge-disproportionated state of the whole RNiO3 perovskite family. The splitting is five times larger for LuNiO3 [2.5(1) eV] than for PrNiO3 [0.5(3) eV], suggesting that the charge transfer between Ni3+delta and Ni3-delta decreases by approaching the itinerant limit and is larger for the heavier lanthanides than suggested in previous studies. The spectroscopic signature of the two Ni sites remains visible above the metal-insulator transition, in agreement with the persistence of dynamic Ni3+delta/Ni3-delta charge fluctuations in the metallic phase. This last result generalizes the occurrence of charge disproportionation as alternative to Jahn-Teller distortions to the dynamic regime, giving further support to recent theoretical work [I. I. Mazin, D. I. Khomskii, R. Lengsdorf, J. A. Alonso, W. G. Marshall, R. M. Ibberson, A. Podlesnyak, M. J. Martiacutenez-Lope, and M. M. Abd-Elmeguid, Phys. Rev. Lett. 98, 176406 (2007)].