Maybe some time after the wedding, you could write a letter to your grandparents and express your gratitude to them for showing you, yet again, that you made the right decision to DA from the JWs, considering that they cannot show love and reasonableness that you have come to expect from real Christians. There's nothing quite as humbling for a JW than to hear that they have failed in being loving, when love could have motivated them to very simply politely decline your invitation out of deference to their beliefs. No, they had to go beyond that, and deliver the one-two punch to the gut in order to make their point.
Excellent point Scully! It never fails to amaze me how Witnesses smugly think they are the most loving people on earth, yet they shun their own flesh and blood because they don't have the same beliefs. The only thing more irritating to a Dub, unless it's when they're lumped in with other cults.