Rare-earth nickelates (chemical formula RNiO₃, R being a rare-earth cation) display a temperature-dependent metal-to-insulator transition (MIT) together with a breathing distortion of the NiO₆ octahedra units at a temperature ranging from 0 to 600 K depending on the size of the R cation. Their rich phase diagram is also characterized by a paramagnetic to antiferromagnetic transition that occurs at the same temperature as the MIT for R = Pr, Nd, while it arises at lower temperatures for all the other members of the series. In this work, we have investigated the order of the MIT in a portion of the phase diagram spanning from SmNiO₃ to NdNiO₃ by means of temperature dependent transport measurements and resonant elastic x-ray scattering performed on high quality epitaxial Sm_xNd_1−xNiO₃ solid solution thin films. Our results show that the order of the metal-to-insulator transition does not depend on whether or not the MIT is coupled with the magnetic transition.