What people tell me is that often when the distance sibling leaves the parent to go back home, the mindset of the sibling is that the caregiver is overly negative. Things really aren't so bad.
I'm not picking on distance caregivers, by the way. A good friend of mine traveled a great deal to cope with her mother's illness and death, because the adult child in town "couldn't deal with it," and things really weren't "that bad." However, distance is often a realistic factor.
The other type of sibling denial is where the sibling turns her or his back and ignores the daily grind that life has become for the one adult child who is doing all the errand running, doctor appointments and emergency handling. The absent sibling acknowledges the parent is ill, but ignores the fact that caregiving takes a huge toll on the caregiver. The distance sibling doesn't want to investigate ways to help, since it's easier to ignore the stress and exhaustion of their hard-working hands-on sibling. Sometimes they simply don't know how to help.
How do we wake up these experts in denial? First, we look at ourselves. Yep. We all have to do that. Have we asked for help directly and with specific requests? I know. We shouldn't have to do that. The siblings should want to pitch in. However, some people just aren't good at figuring out how they can help, and since the caregiver doesn't complain or ask for help, they just go their merry way and don't bother finding out what they can do. So first and foremost, caregivers need to ask for help in general, but also be specific if necessary. Say, "I don't have time to handle the monthly bank statements. Could you take that on? And the taxes, too? That would take a load off of me."