We want to isolate the effects of doping, charge localization and local distortion on the transition from insulating antiferromagnet to superconducting metal in YBCO. We choose Y_{1-x}Eu_xBa_2Cu_3O_{6+y} (0<y<0.4, x=0.08) since the Eu substitution for Y introduces structural disorder in the adjacent CuO_2 layer, due to the difference of the ionic radii, with no change in the Coulomb interaction. The aim is to distinguish distortion-driven and Coulomb-driven disorder in the electronic cluster glass state around the metal-insulator transition (i.e. the distruption of antiferromagnetism and the emergence of the superconductivity) by comparing this isovalent cation disorder with our previous data on the pristine YBCO and on the etherovalent substituted Y_{1-x}Ca_xBa_2Cu_3O_{6+y}.