Backside = Po!! Hier, reverse.
Widmen can = devoted, hier dedicated
Zum ewigen Andenken an deine liebe Schwaegerin -- Von deinem Schwager ...surely the first must be "meine" - to the eternal memory of my sister-in-law from your brother-in-law Oskar. To the eternal memory of your sister-in-law ... from your brother-in-law macht keinen Sinn.