Inorganic lead-free halide double perovskites have recently emerged as possible candidates for potential solar cell absorber and efficient light emitter for light emitting device (LED), with much better stability. Here, we investigate electronic, optical and lattice dynamical properties of double perovskite nanocrystals of Cs2AgIn1-xBixCl6 (x=0, 0.25, 0.5,0.75 and 1). The entire series crystallizes in the cubic structure (space group Fm-3m), as confirmed from XRD and TEM studies. The series transform from indirect (x > 0.25) to direct (x < 0.25) band gap, showing much higher absorption cross-section and lower scattering induced trap states for the later. Careful analysis of Raman spectra suggests the existence of three types of active modes of vibration; 2 T2g modes (one for translational motion of ‘Cs’ and other for octahedral breathing), 1 Eg and 1 A1g mode for stretching of Ag-Cl octahedra. Additionally, we discussed about the carrier-lattice coupling, particularly from longitudinal optical phonons (LO) via Fröhlich interaction, a well-known dominant source of carrier scattering these materials. Simulated Frohlich coupling constant along with our qualitative analysis, point towards dominant carrier-phonon scattering via Frohlich mechanism near room temperature. Simulated mobility limit suggests increase in carrier (hole) mobility with increasing Bi doping, largely due to the decrease in hole effective mass associated with increase in Bi-s orbital character in valence band maxima. Overall, our experimental observations corroborate with the findings of first principles simulations. We believe these results will guide future researchers to pursue further investigations and help in finding the critical Bi concentration in Cs2AgIn1-xBixCl6, which is most appropriate for related applications.