Although silicon oxynitrides are important semiconductors for many practical applications, their second-order nonlinear optical (NLO) properties have never been systemically explored. Using the first-principles calculations, in this article, we discover that the sinoite (e.g., silicon oxynitride Si2N2O) can simultaneously exhibit wide optical band gap, strong second-harmonic generation (SHG) effect, and large birefringence, which are further confirmed by our preliminary experimental measurement. Importantly, we propose that alloying engineering can be further applied to control the balanced NLO properties in the Si2N2O system. Based on first-principles cluster expansion theory, we demonstrate that alloying Ge into Si2N2O can form low formation-energy Si2(1-x)Ge2xN2O alloys, which can in turn achieve controllable phase-matching harmonic output with high SHG efficiency at different energy ranges. Therefore, alloy engineering could provide a unique approach to effectively control the NLO performance of Si2(1-x)Ge2xN2O, making this polar alloy system hold potential applications in tunable laser conversion and controllable optical devices.