This work presents a systematic first-principles investigation of the electronic structure and bandgap modulation in Cu2Ni(Sn,Ge,Si)Se4 kesterites using density functional theory (DFT). Geometry optimizations were performed using the SCAN functional, followed by electronic structure calculations with the hybrid HSE06 functional to ensure high accuracy. Convergence tests were carried out to determine the optimal values of plane-wave cutoff energy (450 eV) and k-point mesh (4×4×2), ensuring precise energy calculations while minimizing computational cost. Our study reveals that the substitution of Sn with Ge and Si leads to a progressive increase in the bandgap, ranging from 0.79 eV (Sn) to 2.35 eV (Si), allowing for fine-tuning of absorption edge energies. The evaluation of the effective masses of electrons and holes relative to the free electron rest mass (m0) showed an increase from 0.25–0.35 m0 (Sn-based) to 0.38–0.50 m0 (Si-based), reflecting a reduction in band curvature near the Fermi level. Spin-polarized density of states (DOS) analysis shows a transition from weakly magnetic behavior in Cu2NiSnSe4 to a non-magnetic semiconductor character in Cu₂NiSiSe₄, highlighting the significant effect of group-IV cation substitution on the electronic and magnetic properties. These findings demonstrate that Cu2Ni(Sn,Ge,Si)Se4 kesterites, with their tunable bandgaps and transport properties, are promising candidates for next-generation thin-film solar cells. Compared to conventional CZTS-based kesterites, the incorporation of Ni and the substitution of Sn with Ge and Si not only enable magnetic and bandgap modulation but also expand their application range, making them suitable for use in photovoltaics spanning from near-infrared to visible light. The tunable band gaps and electronic structures of Cu2Ni(Sn,Ge,Si)Se4 make these materials ideal candidates not only for single-junction solar cells, but also for tandem architectures and infrared photodetectors. In particular, Cu2NiSnSe4 may serve well in IR-sensing, while Cu2NiSiSe4 is optimized for visible-light photovoltaic absorption.