In this work, we prepared SrBi2(Ta1−xNbx)2O9 (x = 0, 0.5, 1.0) and Bi2SiO5-added SrBi2(Ta1−xNbx)2O9, and then investigated their ferroelectric properties, crystal, and electronic structures. P-E hysteresis-loop and dielectric-constant measurements demonstrated that the Nb substitution and the Bi2SiO5 addition increased the remanent polarization and the Curie temperature although these composition changes made the coercive field higher slightly by the substitution and the addition. The Rietveld analyses using the neutron diffraction data indicated that the Nb substitution induced higher tilting angles of the (Ta,Nb)-O6 and distorted the octahedra. Larger distortions within the perovskite layer were also expected from electron-density distributions visualized by the maximum-entropy method using synchrotron X-ray diffraction. These structure changes can be considered as one of the reasons for improvement of the electric properties.