When running qmake followed by make sis for SUBDIRS projects, the following error occurs:
make -s -f Makefile fail_sis_nopkg
Makefile:106: *** PKG file does not exist, 'SIS' target is only supported for executables or projects with DEPLOYMENT statement. Stop.

This is currently being worked around by including the targets of sibling projects by explicit deployment statements.

e.g. autotests that need to launch subprocesses
e.g. fluidlauncher demo

It's a bit messy, and would be better if the build system could do this for you.