To me, it seems like Grandma is mad that her grandson is getting married to solidify a relationship she's never approved of. She sounds like all she'll say to this couple their whole lives is stuff like, "Happy 10th anniversary. Too bad your relationship began in such sin!" "What a beautiful great-granddaughter I have, and I'm simply shocked you didn't give me an illegitimate one when you were 17, too!"
In other words, she's dwelling on the past she disapproves of rather than the joys of the present. I think she certainly doesn't have to be happy about the wedding, as is her right, but she sounds like someone who would only offer passive-aggressive congratulations at best. "Oh I'm so happy for you!" said through clenched teeth doesn't exactly convey happiness!