The aim of the present proposal is to relate the macroscopic magnetic properties with the nuclear and magnetic structure of a series of Ni48Mn36In16-xSnx Heusler alloys. These alloys show an unpredictable trend of the Curie temperature of the austenitic phase with x, highlighting the existence of different mechanisms controlling the saturation magnetization and the Curie temperature of Mn-based Heusler alloys. A deep understanding of these mechanisms, pivotal to optimize this class of multifunctional materials, requires the synergic use of different experimental techniques and first-principles calculations. High-resolution neutron diffraction experiments are essential to: clarify the distribution on the different sites of the Mn atoms, determine the magnetic moment on the Mn and Ni sites and verify the possible presence of short-range antiferromagnetic interactions.