There is no such thing as grandparents rights, neither are there parental rights ~ all the rights are with the children, in law.
It is your granchilds right to contact with her family which is the issue, and the only way you can get hlafway to solving this is through the court ~ which can be expensive!
If you have tried all avenues besides the courts, I would suggest a meeting with the CAB or maybe a free half hour with a solicitor. They can advise as to whether you have a case, and where to go from there.
Good luck x