when PIN is high, you need INa to be 0 and INb to be 1
When PIN is low, you need INa to be 1 and INb to be 0?
Use an inverter, drive the inverter with PIN, drive INb with PIN, and drive INa with the inverter.
If you're using any RS-232 drivers on the board you can use a spare channel (driving the rs-232 inputs) as a logic level inverter. Alternatively you can use a regular inverter chip (is it a 7404? but they're big) or make your own out of an NFET and a pullup, or an NFET and a PFET