In this paper, we present the temperature and field dependent neutron diffraction (ND) study to unravel the structural and magnetic properties in the Ni₅₀Mn₃₈Sb₁₂ Heusler system. This alloy shows martensitic transition from the high temperature austenite cubic phase to a low temperature martensite orthorhombic phase on cooling. At 3 K, the lattice parameters and magnetic moments are found to be almost insensitive to field. Just below the martensitic transition temperature, the martensite phase fraction is found to be 85%. Upon applying the field, the austenite phase becomes dominant, and the field induced reverse martensitic transition is clearly observed in the ND data.Therefore, the present study gives an estimate of the strength of the martensite phase or the sharpness of the martensitic transition. Variation of individual moments and the change in the phase fraction obtained from the analysis of the ND data vividly show the change in the magneto-structural state of the material across the transition