The functional characteristics of double perovskites with a unique ferromagnetic-insulator ground state have been controversial due to the unavoidable presence of antisite disorders (ASDs). Here, we aim to investigate the origin of magnetic ordering on local and global scales in the Sm2NiMnO6 (SNMO) double perovskite system. Different calcination routes are exploited to generate different cation arrangements in SNMO and the corresponding magnetic configurations are examined using the high-energy (E ??? 0.3 eV) ???hot neutrons???, which has helped to overcome Sm absorption as well as to record total (Bragg + diffuse) scattering profiles with high momentum transfer (Qmax ??? 24 ?????1). We have observed that the Ni-Mn sublattice adopts long-range collinear ferromagnetic FxFz structure with a commensurate k = (0, 0, 0) propagation vector, below ordering temperature T < 160 K, irrespective of variable ASD concentrations. In addition, the signatures indicating the antiparallel polarization of Sm paramagnetic moments with respect to the Ni-Mn network are noticed in the vicinity of anomalous magnetic transitions at T < 35 K. The real-space pair distribution function calculations have provided a direct visualization of ASDs by means of broadening in Ni/Mn-Mn/Ni linkage. Employing the reverse Monte Carlo approach on diffuse magnetic scattering profiles, we have observed the negative spin-spin correlation function which suggests the Ni-Ni antiferromagnetic exchange interactions ranging up to first-nearest-neighbor distance. These results confirm that the existence of ASDs in a cation ordered host matrix leads to competing ferromagnetic-antiferromagnetic phases in a broad temperature range, which quantitatively governs the temperature-dependent bulk magnetic observables of the SNMO system.