Help with drain and vent configuration
You're  going to need to replace the existing cast iron tee with an elbow (or just plug the top), and install a sanitary tee lower in the galvanized pipe to connect the bathroom drain. I probably just cut off the galvanized down near the floor and use a no-hub coupling to run new PVC to the sinks. 

The next question is what to do with that HVAC register.