  Structural, magnetization and heat capacity measurements were performed on
Ce$_2$(Pd$_{1-x}$Ni$_x$)$_2$Sn ($0 \leq x \leq 0.25$) alloys, covering the full
range of the Mo$_2$FeB$_2$ structure stability. In this system, the two
transitions observed in Ce$_2$Pd$_2$Sn (at $T_N=4.8$\,K and $T_C=2.1$\,K
respectively) converge into a tri-critical point at $T_{cr}\approx 3.4$\,K for
$x\approx 0.3$, where the intermediate antiferromagnetic AF phase is
suppressed. The $T_N(x)$ phase boundary decrease is due to an incipient Kondo
screening of the Ce-4f moments and local atomic disorder in the alloy. Both
mechanisms affect the formation of Ce-magnetic dimers on which the
Shastry-Sutherland lattice (SSL) builds up. On the contrary, the $T_C(x)$
transition to the ferromagnetic ground state increases as a consequence of the
weakening of the AF-SSL phase. Applied magnetic field also suppresses the AF
phase like in the stoichiometric compound.
