This first principles study provides the structural and magnetic properties of Ni2Mn1.5Sn0.5 alloy, focusing on the austenitic and martensitic phases with ferromagnetic (FM) and ferrimagnetic (FiM) ordering. The cubic L21 austenite structure exhibits both FM and FiM states close in energy (∆E ≈ 5 meV/atom), however, the FiM state is energetically preferred in austenite. The martensitic phase is described by the FiM orthorhombic 4O structure, which is found to be preferable over 30 meV/atom as compared to L21 structure. It is shown that the atomic configurations of Mn excess atoms in Sn sublattice underscores the critical role in the prediction of ground state. It is concluded that structures with random Mn distributions are more energetically favorable than ordered configurations, emphasizing the role of Mn atom arrangement in determining magnetic properties and energy stability.