Common mode problem with floating sensor
The normal method is to tie one or both inputs to the mid rail (0 volts) of the instrumentation amp via typically 100 kohm resistor. If your sensor has reasonably low output impedance this won’t create an error due to 100 kohm mismatch in value. To supplement this (should common mode input noise be a problem) then a capacitor in parallel with each resistor is typically used. Maybe 1 nF to 100 nF won’t kill your bandwidth too much?