Dee,
As a kid growing up I was like Larry's son G, the one that remembered Dad's special days. My 1/2 brother and 1/2 sister never did a thing. I would get cards and mail to them w/ a Stamped return envelope so they could mail Dad a card. Sometimes I would send poetry about loosing our elders, that worked for a few times to wake them up.. finally I gave out on this as Dad always seemed more thrilled with their measley card, since neither lived with him and I did...as his cow hand, ranch foreman, driver when he was too intoxicated, secretary, cook, housekeeper etc....I could give him actual gifts & cook special meals & he still was more pleased with the other 2's card that I sent for them to send! I finally had a talk with them, & told them either they remembered on their own or I would let them look like the A-hole people they were, that they were hurting Dad by not honoring him on special days, for a while this worked. At least one or both of them usually remembered. Sadly they really got into recalling as Dad aged and the end seemed near. Not that they were making up for lost time, but they wanted to be sure they were in good standing for the inheritance. Long tale to get to this, could Larry's son G talk to them? Coming from another sibling it might have more impact? Good Luck
PS None of my Dad's kids had the same Mom.