Thermoelectric materials require complex microstructures to optimize the power conversion efficiency. Grain boundaries (GBs) reduce the thermal conductivity in nanocrystalline materials but often also reduce the electrical conductivity. We have recently shown that elemental segregation at GBs can make them electrically conductive or non-resistive and thereby improving the thermoelectric properties. Nevertheless, the doping elements participate in the chemistry within the grain matrix, which can limit the design space. In this work, we present an independent control of doping to the GBs. Specifically, we add InSb to Nb_0.95Ti_0.05FeSb to selectively modify the chemistry of the GBs without increasing the carrier concentration of the matrix. Using high resolution transmission electron microscopy and atom probe tomography, we understand the role of InSb in the selective modification of the chemistry of the GBs and we establish structure-property relationships between the chemistry of the GBs and their electrical behaviour. Incorporating InSb into the GBs successfully increases power factor and zTof fine-grained Nb_0.95Ti_0.05FeSb. We demonstrate that the negative impact of GBs in the power factor can be overcome by GB engineering.