Light-emitting complex defects in silicon have been considered a potential platform for quantum technologies based on spin and photon degrees of freedom working at telecom wavelengths. Their integration in complex devices is still in its infancy, and it was mostly focused on light extraction and guiding. Here we address the control of the electronic states of carbon-related impurities (G-centers) via strain engineering. By embedding them in patches of silicon on insulator and topping them with SiN, symmetry breaking along [001] and [110] directions is demonstrated, resulting in a controlled splitting of the zero phonon line (ZPL), as accounted for by the piezospectroscopic theoretical framework. The splitting can be as large as 18 meV and it is finely tuned by selecting patch size or by moving in different positions on the patch. Some of the split, strained ZPLs are almost fully polarized and their overall intensity is enhanced up to 7 times with respect to the flat areas, whereas their recombination dynamics is slightly affected. Our technique can be extended to other impurities and Si-based devices such as suspended bridges, photonic crystal microcavities, Mie resonators, and integrated photonic circuits.