The Arduino build preprocessing cannot locate sensor source and header when it is located in another directory. The code needs to be restructured.

    --Sketchbook
    ---NodeIntercom
    ----NodeIntercom.ino
    ---MasterNode
    ----MasterNode.ino
    ---libraries
    ----Sensor
    -----Sensor.h
    -----Sensor.cpp

The Sensor header is included with

    #include <Sensor.h>

in the sketches that need to use it.