The lattice parameters and magnetic properties of (R = Ho, Er) have been investigated. The deviation of the lattice parameters from Vegard's law indicates that the Ce valence in both systems fluctuates with the concentration of the dopant R. This fluctuation is directly relevant to the degree of localization of Ce 4f electrons, and is independent of the atomic size of the dopant element. The observed metamagnetic transition of has been analysed from the description in the band picture. The Ce valence fluctuation from the mixed-valence state to the localized state increases the moments of the Ce and Fe ions, and the Curie temperature. The changes in the mixed-valence state also affect the properties of the Ce-Fe coupling.