Mg₂La₃Sb₃O₁₄ and Ca₂La₃Sb₃O₁₄ are rhombohedral pyrochlores. A detailed structural investigation shows that, as a consequence of the larger difference in the ionic radii of La³⁺ and Mg²⁺ with respect to La³⁺ and Ca²⁺, in the former compound there is an ordered distribution of the cations and a local inversion symmetry for La³⁺ sites. On the contrary, because of the cationic disorder observed in Ca₂La₃Sb₃O₁₄, breaking of the local inversion symmetry at La³⁺ is observed. This very peculiar structural behavior has a big impact on the Eu³⁺ luminescence spectroscopy of the doped materials Mg₂La₃Sb₃O₁₄:1% Eu³⁺ and Ca₂La₃Sb₃O₁₄:1% Eu³⁺. Moreover, first-principles calculations confirm the structural evidence, and elucidate the structure-spectroscopy relationships of these Eu³⁺-doped materials.