on the back of card without adress:
Zum ewigen Andenken an meine liebe Schwägerin gewidmet im Kriegsjahr 14/18 (1914-1918 meant?)/
to the eternal memory of my dear sister-in-law dedicated in war-year.
Von deinem Schwager Oskar/your brother-in-law Oskar
card with adress:
Zum Andenken von deinem Onkel/in memory to your uncel
gewidmet im Kriegsjahr 14/18/devoted in war-year ...
(the name is unreadable for me but looks for another person)
date is 16.3.or 7.1918 (16th March or July 1918)
did your father live in time-period 1904-1975 in Vienna?
so you could ask at Historical registry-dates (is free) to get his adress that time and persons who lived also in same household and maybe some more info:
Historische Meldeunterlagen (1904-1975) - Auskunft
take e-mail-adress at "Zuständige Stelle"http://www.wien.gv.at/amtshelfer/kultur/archiv/forschung/his...
you also can ask for the person stated with the adress. this would also make much sense.
Fritzi(= Friederike) Reiß
Währinger Gürtel 137/3