Is it a sin to ignore your grandfather?

If he is incoherant and doesn't even know who you are half the time, I wouldn't worry about it. Besides, I wouldn't worry about sin and god and stuff like that. If you believe in that, and have lived a good life, everything will be fine in the end. You should not ignor your grandfather from fear of god, but you should not ignore him because you should respect your elders, no matter how old and rambling. Because one day he will pass away, and although it may not be an annoyance to have to see him anymore, you will still miss him and wish you had spent more time with him. Good luck! And I wouldn't worry too much about it if you live a good life otherwise.

I am not very well versed on the Bible, so I can't say if it's a sin to God. However, I feel that it is wrong. He is your family, and he is an elderly man with health problems. He deserves the recognition and respect of his family. Even if he talks incessantly and you don't know what he is talking about, and even if he doesn't know what he's saying most of the time, you should have the respect to listen to him. On some level I believe that people like this know that they are being treated poorly. I always say that you should treat other people as you would want to be treated. Would you want to be old and all alone in the world, knowing that even your family was avoiding you and didn't want to bother with you? Spend some time with him. You don't know how many more chances you will have to make him happy.