Just reporting back now that sister's Anniversary is nearly here. I bit the bullet and phoned her son and daughter, neither of whom had given the anniversary a thought but both thanked me for the head's up and said they would talk to each other and decide what to do.
Yesterday, being DH's day at day care, I treated Sister to lunch in her current favourite restaurant and gave her an exotic Golden houseplant. We chatted about her wedding and honeymoon and all the funny things that happened then.
She told me that out of the blue her son had suggested taking her out for dinner on Friday and her daughter had invited her to go up to stay with her and the DGC for a long weekend - a very rare occurrence, her DD said part Anniversary, part Mother's Day treat. She said she was incredibly touched that they had remembered (I just agreed that it was wonderful) as they are usually not good about marking events, and she was glad she would not be spending these days alone.
Thank you all for your suggestions, which have led to this happy conclusion.