Your feelings are completely understandable and you do have legal rights. The law prefers the intact biological parents over all others. In the case of one grandparent against another grandparent, the legal gloves come off.
What you need to do is to hire the most agressive trial lawyer in 3rd party child custody cases you can possibly find (and afford) who practices in the court where the current custody order was issued (or, if no order exists, then where the children have resided for the last 6 months).
It isn't that I advocate running up legal fees when they are not necessary, because I don't. I really, really don't. But, where as here neither parent has the child (children) and one set of grandparents is totally excluding another set of grandparents, someone really needs to step forward on behalf of the children -- to make sure that they know both of their parents and both sides of their family and know that they are never alone in the world. IF you think that you have some or all of that to offer to this child (or children), then hire that lawyer and file that case!
This response does not create an attorney-client relationship and is intended for general information purposes only.