  We report neutron diffraction experiments performed in the tetragonal
antiferromagnetic heavy fermion system CeRhIn$_{5-x}$Sn$_{x}$ in its ($x$, $T$)
phase diagram up to the vicinity of the critical concentration $x_c$ $\approx$
0.40, where long range magnetic order is suppressed. The propagation vector of
the magnetic structure is found to be $\bf{k_{IC}}$=(1/2, 1/2, $k_l$) with
$k_l$ increasing from $k_l$=0.298 to $k_l$=0.410 when $x$ increases from $x$=0
to $x$=0.26. Surprisingly, for $x$=0.30, the order has changed drastically and
a commensurate antiferromagnetism with $\bf{k_{C}}$=(1/2, 1/2, 0) is found.
This concentration is located in the proximity of the quantum critical point
where superconductivity is expected.
