  Using first-principles based Density Functional Theory (DFT), we have
investigated the structural instabilities in the austenite phases of
Mn$_{2}$Ni{\it X} ({\it X}= Al, Ga, In, Sn) magnetic shape memory alloys
(MSMA). A complete softening is observed in the acoustic TA$_{2}$ branches for
all the materials along [$\xi\xi$0] directions leading to the instability in
the austenite structure which effectively stabilizes into martensitic
structure. The reasons behind this softening are traced back to the repulsion
from the optical T$_{2g}$ branches and to the nesting features in the Fermi
surfaces. The vibrational density of states, the force constants and the
elastic moduli are also computed and analyzed, which reconfirm the underlying
mechanism behind the instabilities. The results indicate that the phonon
anomalies are related to the occurrence of possible pre-martensitic phases
which can be quite complex.
