With children of that age, I think you would do well to respect their decision. I realise how hard this will be, especially as you feel that their mother has persuaded them into believing that they do not want contact with their father or his parents.
Try to look at this from the girls' angle and what may be, obviously, I am guessing here, be their point of view..
Their parents have divorced - something that most children fail to understand and which they do resent.
They live with their mother, so in her house and obeying her rules. Any sensible child in such circumstances opts for a quiet life, something best achieved by doing as mummy says.
The eldest is old enough to contact you if she wants to do so, and her sisters will soon be old enough too. If you try to exert pressure in order to see them, you will only cause resentment along the lines of "gran's making our lives even harder than they already are" and this will result in none of the girls being willing to contact you, or to try to persuade their mother into letting them see you or their dad.
So possess you soul in patience, and sent the eldest granddaughter a letter on her 18th birthday telling her that you hope that you could meet up, now that she is an adult.
I realise two years is a long time to wait, but honestly, what else can you do? Making a difficult family situation worse certainly will not help.