This work explores the influence of Co substitution in place of Mn and Ni on the structural, thermal, and magnetic properties of Mn45Ni35Sn15Co5 (C1) and Mn50Ni30Sn15Co5 (C2) samples. The samples were prepared using the green compact method, annealed in a vacuum-sealed quartz tube at 950 ◦C for 96 h, and quenched in cold water. X-ray diffraction analysis confirmed the presence of Mn2NiSn with the XA (inverse Heusler) structure as the major phase. Differential scanning calorimetry revealed possible magnetic transitions at higher temperature regions. Magnetic parameters, including the spin-wave stiffness, exchange stiffness and magneto crystalline anisotropy were estimated using modified Bloch's law and the law of approach to saturation. The isothermal magnetization analysis revealed that the samples are soft ferromagnetic in nature. Co substitution of Mn increases the magnetization, spin-wave stiffness and exchange stiffness. Whereas Co substitution of Ni gives rise to antiferromagnetic sub lattices and frustrated magnetic state.