  A delicate balance between various factors such as site occupancy,
composition and magnetic ordering seems to affect the stability of the
martensitic phase in Mn$_{2}$Ni$_{1+x}$Sn$_{1-x}$. Using first-principles DFT
calculations, we explore the impacts of each one of these factors on the
martensitic stability of this system. Our results on total energies, magnetic
moments and electronic structures upon changes in the composition, the magnetic
configurations and the site occupancies show that the occupancies at the 4d
sites in the Inverse Heusler crystal structure play the most crucial role. The
presence of Mn at the 4d sites originally occupied by Sn and it's interaction
with the Mn atoms at other sites decide the stability of the martensitic
phases. This explains the discrepancy between the experiments and earlier DFT
calculations regarding phase stability in Mn$_{2}$NiSn. Our results
qualitatively explain the trends observed experimentally with regard to
martensitic phase stability and the magnetisations in Ni-excess, Sn-deficient
Mn$_{2}$NiSn system.
