I hope this isn't too late but I would advise you to go,I have been here long enough to know that occasionally people with dementia do regain some cognition at the end of life and even if she doesn't recognise you she may recognise that the person holding her hand is someone who loves her,plus as you say for your grandad so he will know someone was with her for a while.But it is up to you,no one here would cticise you if you decide not to go its a very difficult decision,I don't know anything about other family but having someone go with you to support you would help if there is someone who would do that.