A detailed investigation on the relative isotopic distributions has been carried out for the first time in case of even-even correlated fission fragments for the ²³⁵U(n_th, f) fission reaction. High-statistics data were obtained in a prompt γ-ray spectroscopy measurement during the EXILL campaign at ILL, Grenoble, France. The extensive off-line analysis of the coincidence data have been carried out using four different coincidence methods. Combining the results from two-dimensional γ-γ and three-dimensional γ-γ-γ coincidence analysis, a comprehensive picture of the relative isotopic yield distributions of the even-even neutron-rich fission fragments has emerged. The experimentally observed results have been substantiated by the theoretical calculations based on a novel approach of isospin conservation, and a reasonable agreement has been obtained. The calculations following the semiempirical GEF model have also been carried out. The results from the GEF model calculations are found to be in fair agreement with the experimental results.