Also to put your mind at ease, here is the statutes from the SC Domestic Relations Code:
33) to order periods of visitation for the grandparents of a minor child where either or both parents of the minor child is or are deceased, or are divorced, or are living separate and apart in different habitats regardless of the existence of a court order or agreement, and upon a written finding that the visitation rights would be in the best interests of the child and would not interfere with the parent/child relationship. In determining whether to order visitation for the grandparents, the court shall consider the nature of the relationship between the child and his grandparents prior to the filing of the petition or complaint
The bolded and underlined is very important. Your father's mother has blatantly made threats to interfere with your relationship with your child by trying to use the Law to scare you and intimidate you into letting her the children. THAT is wrong and can be brought that the petitioner has made threats in order to make you accede to her wishes.
Think of it this way, from a legal standpoint, the grandmother has to file for visitation and/or custody in YOUR home state where the children reside. Is grandma going to waste time and money to do that? Even if she does, she will have to show such a huge burden of proof, its almost pathetic if she does try.
From a legal standpoint I would not think you have much to worry about but from a personal one, I can see where you are going to be aggravated.