Thank you for your letter -- and I know what you mean. I don't think I've ever visited a nursing home without having the staff ask me to greet people who never have anyone come see them.
I realize there may be practical reasons why some relatives aren't able to visit their loved ones as often as they'd like (such as disability or distance). But often that's not the case, and even a brief visit could do wonders for an aging relative who feels lonely or even abandoned. Even if their memory has faded and they might not be able to remember the visit, it's still important to see that they're receiving good care.
I often think of Jesus' words in the Sermon on the Mount, urging us to treat others the way we'd want to be treated if we were in their situation. We often call it the Golden Rule: "In everything, do to others what you would have them do to you" (Matthew 7:12). This should be applied to every relationship -- but especially to our loved ones.
May I suggest something? The next time you visit your sister, ask God to lead you to at least one other person you can encourage with a visit. Listen to them; talk with them; pray with them as you have opportunity. God can use you not only to cheer them up, but also to point them to the hope we can have of eternal life because of Jesus.