Who knows: does a Delta parent have S2's these days? FIL was paranoid about standby so he wouldn't use pass unless with my hubby to be. (didn't like that either) That's when I took the companion role on his pass until marriage.
Are there Delta grandchildren (nondependents) in the picture? If not, you have 6 S2s/year and they are helpful for newbies and popular destinations.
Winter in EU is great--just watch out for holidays and the Christmas markets. Many EU cities are seasonal--Pisa, Malaga, Prague, IST, etc. Oh yeah--they can be VERY cold.
The key being flexible and having a Plan B (or C)