The best subjects to talk about with your fiance's grandmother (widowed and part of the 'silent generation')?

Just treat the poor woman like a human being and you may find that she's a lot more fun than you think she is. If being with her is a chore for you it will make things more awkward than they are already. Talk to her about the things that interest her now and let friendship develop from there. Don't ask her about being evacuated unless you are really interested, likewise the royals, Marilyn etc., if you are not interested that is a little patronising and won't help. You don't stop being human when you reach the age of 60.