Upvote:1
As mentioned in the comments, if you have sufficient proof that you are related by blood to them and they are your grandparents, their not mentioning this in their earlier documents will just be considered an error. It can be considered a problem but certainly not for you.
Your birth certificate and other documents are perfectly valid for this purpose as well. Otherwise another way is to simply apply for a tourist visa and get a letter from your grandparents which says that you will be visiting them as a tourist. Make sure that you are able to fulfill all the necessary requirements for a tourist in this case then.