Interfaces in polymer–matrix composites subjected to coupled hygrothermal environments exhibit strongly nonlinear and scale-dependent responses that govern global durability, particularly when additional heterogeneities such as embedded sensors perturb the local morphology. This work introduces a multiscale and multiphysics framework designed to capture hygrothermally induced degradation processes at sensor–matrix and matrix–fiber interfaces in PA6/GF30 composites used for hot-water storage applications. At the structural scale, the composite material and PA6 liner are represented by homogenized media to capture the long-term evolution of temperature and moisture fields under service conditions. A coupled hygrothermal model based on boundary-layer theory and the analogy between Fickian mass diffusion and heat convection is proposed, incorporating consistent swelling and thermal dilation effects. These macroscale fields are transferred to a mesoscale subdomain through kinematically compatible submodeling to recover the local gradients that cannot be resolved at the structural level. The mesoscale model explicitly describes the heterogeneity of the composite, including randomly distributed short glass fibers and an embedded optical fiber. This resolution reveals how hygrothermal aging induces nonuniform moisture uptake and evolving swelling mismatches, generating localized stress intensifications near the sensor and within the microstructure. The simulations show that hygrothermal exposure creates significant moisture gradients that lead to incompatible swelling strains. These incompatibilities produce pronounced stress amplifications at the sensor–matrix interphase and at matrix–fiber interfaces, far exceeding homogenized predictions and demonstrating the necessity of a multiscale treatment. To characterize the resulting degradation mechanisms, a mixed-mode cohesive formulation is introduced along the interfaces. The results indicate that hygrothermal ageing modifies the traction–separation response by redistributing energy release rates toward debonding modes activated earlier than in dry conditions. The framework highlights the critical role of sensor-induced morphological singularities and hygrothermally driven interfacial mismatch as primary drivers of decohesion. The proposed framework provides a rigorous basis for predicting aging-controlled interfacial failures, assessing the durability of sensor-integrated type IV composite pressure vesseels.