  The application of pressure as well as the successive substitution of Ru with
Fe in the hidden order (HO) compound URu$_2$Si$_2$ leads to the formation of
the large moment antiferromagnetic phase (LMAFM). Here we have investigated the
substitution series URu$_{2-x}$Fe$_x$Si$_2$ from $x$\,=\,0.0 to 2.0 by U\,4$f$
core-level photoelectron spectroscopy and have observed non-monotonic changes
in the spectra. The initial increase and subsequent decrease of the spectral
weight of the 4$f$ core level satellite with increasing $x$ stands for a
non-monotonic 5$f$ filling across the substitution series. The competition of
chemical pressure and increase of the density of states at the Fermi energy,
both due to substitution of Ru with Fe, can explain such a behavior. An
extended Doniach phase diagram including the $x$ dependence of the density of
states is proposed. Also in URu$_{2-x}$Fe$_x$Si$_2$ the ground state is a
singlet or quasi-doublet state consisting of two singlets. Hence, the formation
of magnetic order in the URu$_{2-x}$Fe$_x$Si$_2$ substitution series must be
explained within a singlet magnetism model.
