Cellulose nanocrystals (CNCs) are increasingly utilized to enhance the interphases in glass fiber-reinforced polymer composites. Improved adhesion at glass fiber-epoxy interfaces has been demonstrated experimentally when CNCs are applied as a coating on glass fibers. However, fundamental insights into the bonding mechanisms between CNCs and glass fibers are still lacking. This study employs molecular dynamics simulations to investigate the structure-property relationships at the CNC-glass fiber interface. The effect of silane sizing, a protective coating used on commercial glass fibers, is examined by incorporating covalently bonded silane molecules onto the glass fiber surfaces. A CNC particle is deposited onto the glass fibers in two different orientations, and adhesion and frictional properties are analyzed by simulating normal and shear deformations. The simulation results reveal that silane sizing increases interfacial adhesion and frictional forces by approximately 2.6 and 5 times, respectively, compared to models without silane coating. Minimal impact from CNC orientation is observed. Contrary to the expected role of hydrogen bonding in enhancing interphase properties, a silane coating is shown to reduce interfacial hydrogen bonds by about two-thirds, suggesting that hydrogen bonding is not the primary factor in improving adhesion and friction. Instead, it is observed that a mono-layer silane coating increases the interpenetration depth between a CNC and glass fiber from about 0.1 to 0.8 nm, indicating a significant enhancement in mechanical interlock. This enhanced interlock physically stabilizes a CNC particle, improving interphase adhesion and friction while increasing interfacial bonding. These findings provide a basis for further optimization of interphase properties and mechanical responses in glass fiber-reinforced composites through CNC coatings.