Niobates of the formula Ba2LaNbO6 and BaLaM2+NbO6 (M2+ = Mg, Ca) were synthesized by the conventional solid state route. Size dependence of M-cation on the bulk crystal structure of BaLaM2+NbO6 was studied and compared with the Ba2LaNbO6 compound. X-ray diffraction study confirmed that BaLaMgNbO6 compound has the rhombohedral (R3 over bar ) crystal structure as compared to the monoclinic (I2/m) in BaLaCaNbO6 and Ba2LaNbO6. The change in the local structure of La cation among these compounds was investigated by carrying out Photoluminescence study on 2 atom% Eu3+-doped samples. PL study of BaLaMgNbO6:Eu3+ sample indicates Eu3+ ions occupying the distorted 12-coordinated A-site, while in Ba2LaNbO6: Eu3+, Eu3+ is present at highly symmetric octahedral B-site. Upon excitation, the light emission of these compounds changes from reddish-orange to red to purple in the order Ba2LaNbO6:Eu -> BaLaCaNbO6:Eu -> BaLaMgNbO6:Eu3+, due to change in Eu3+-ions site occupancy. Lifetime study also confirmed the presence of two different Eu3+ components at two different lattice sites and their respective emission spectra were isolated by time resoled emission spectroscopy. Furthermore, this site selective lattice occupancy of Eu3+ ions also gave various new insights about its radiative and nonradiative properties at different lattice sites. This works presents a complete structural understanding of BaLaMNbO6-based matrices and their versatile phosphor characteristics when doped with Eu3+ ion.