That's also the same thing that happened to me, though it happened while I'm in Al Simhara.
My family had 3 sons, I let them accompany their parents. The dad was continuing a tomb exploration, when we arrived back at our hometown. Poof! Two of the son's icons became green, unclickable and unselectable, while the other one was totally gone. But, it's kinda weird, cause the game announced that it will be the son's birthday in a few days. So I guess, the game still acknowledges them as part of the family.
too bad... I already collected fragments for a sarcophagus, and it's actually a legacy family that I am playing, I think, it's because, I let one of their sons join the travel. The parents were not yet elders when I first brought them to Al Simhara. I read something online, that it's actually a common glitch in the World Adventures. Some say the glitch is because of the family tree mechanics.
*sigh* I love that family so much, I was planning to post online their stories.