The structural, temperature variation of magnetic and dielectric properties of RE2NiMnO6 (RE=La, Pr, Sm, and Tb) double perovskites have been investigated. RE2NiMnO6 have crystallized into pure monoclinic bulk phase with space group P21/n, shows ferromagnetism and relaxor dielectric behavior in intermediate temperatures. Further, magnetic and dielectric properties show a correlation with the radius of RE ions and thereby with the average Ni–O–Mn bond angle in the monoclinic double perovskite structure. These investigations suggest the occurrence of a spin-lattice coupling in RE2NiMnO6 which makes them attractive materials in the field of spintronics.