In high-temperature, adverse environments, the premature failure of environmental barrier coatings (EBCs) is a critical phenomenon that can significantly impact their applications in both aircraft engines and land-based gas turbines. The delamination failure of EBCs typically occurs at the topcoat/TGO or TGO/bond–coat interfaces, primarily due to thermal and shrinkage strains, as well as water vapor corrosion, resulting in crack propagation and coating spallation. This paper presents a systematic study of the degradation of bi-layer disilicate Yb2Si2O7 (YbDS)/Si EBCs using COMSOL Multiphysics methodologies. Thermal-cycle-induced temperature fields were implemented into the EBC model, aiming to simulate the system’s in-service operation. The high-temperature creep models of topcoat YbDS, monosilicate Yb2SiO5 (YbMS), silica TGO, Si bond coat, and SiC substrate are included for the built-up undulated coating geometries. Based on the local stress evolution and distribution in the EBC system during thermal cycles in water vapor environments, and on the transformation from YbDS to YbMS and TGO growth kinetics observed at elevated temperatures, the experimentally observed EBC degradation mode was explained in terms of the simulated results. The J-integral, virtual crack extension, and phase-field damage model were implemented to investigate crack nucleation and propagation under thermal cycles, considering the cristobalite TGO, which undergoes a displaced β-α phase transformation between 220 °C and 270 °C with an associated large volume shrinkage upon cooling. The significant phase-field value of TGO, due to its high tensile stress, leads to the automatic nucleation of cracks in TGO and their subsequent bifurcation and propagation along both YbDS/TGO and Si/TGO interfaces. The linking of these bifurcation-induced cracks during cooling cycles could be the primary mechanism leading to the EBC’s spallation and failure. The simulated TGO crack growth pattern was compared with that experimentally observed in the literature.