suggestion on a complicated inter-process communication

[Way]

Hello friends,

I have a little messy situation on IPC. Please if you can, give me
some suggestion on how to implement. Thanks a lot!

-> denotes create


MainProcess -> Process1 -> Process3 (from os.system)
                   |
                    -> Process2 (from os.system) -> Process4 (from
os.system) ->Process5

I would like to make the communication between Process1 and Process5.
Process1 needs Process5's output to provide argument to generate
Process3, and in turn Process5 needs to wait Process3 finished.

Thank you very much if you can give a hint.