So sad that they are being so incredibly selfish and self-absorbed, but we have seen the same thing with one of my FIL's biological sons. When FIL was diagnosed with esophageal cancer, his son (who supposedly moved to NC from MD to be 'closer' to his Dad) came twice to the hospital (FIL was in the hospital twice for a total of nearly 2 months), and hasn't come to their home or invited them to his in over a year - he's too busy and can't make the time. His son has lived here for nearly four years, and they probably haven't seen him & his family more than a dozen times, really sad because Nan & Pop adore their grandchildren and don't really have a relationship with those two grandkids. They live 40 minutes away.
His daughter, who moved nearby around a year ago, won't come over either. She would want to come over to get away from her mom for a few days (and she is in her 40's), but only wanted to get drunk, download carp to their computer, and they caught her stealing their medications. She got insulted that they set boundaries, and has only spoken with them once in over 6 months.
We are fortunate to live next door to my in-laws, and treasure the time we have with them. I mow their yard. DH & I help with whatever they need around the house. DH is FIL's stepson, but you'd never know it. Pop is his DAD, period. And if it's at all possible, I love him just as much as my DH does, so blood doesn't always matter. Good for your daughter's fiance for stepping up, shows HIS character.
I hope they realize how fast time goes by. I lost my Dad 20 years ago, to a sudden heart attack. He was only 48. We had a great relationship, so I don't have regrets that I should have done 'more'. I am saddened by those who DO know when their loved ones have been told they're on limited time, and don't take every opportunity to show them how much they are loved and cherished. I am sorry you both have to go through this feeling abandoned by your children. 
