What you are talking about is grandparent rights. In general, all but a few states in the US no longer have grandparent rights. In fact, the very concept has been rule unconstitutional. However, since it involves civil law, it will take each state having a case rise that high in the court system, or the legislature overruling the laws, before it will be revoked.
In no state in the US do grandparent rights extend in a situation where an intact, nuclear family exists. So, if legal mother and legal father are legally married, then no grandparents can interfer and attempt to exert control for custody or visitation. In the states where grandparent rights do exist, this does not pertain to any non-intact family aka any configuration other than the aforementioned one.
So, suing for custody may or may not even be an option where you live. She needs to consult an attorney. Lots of angry grandparents can say stuff like this. Only a lawyer can tell you if it's even a possibility in your circumstances.
However, there is also the risk of someone calling Social Services and a child welfare determination that something puts the child at risk. Normally, Social Services tries to not remove children unless absolutely necessary. However, that is not guaranteed.
My husband and I are the legal parents of all of our children and are legally married. Our children are at no risk from outsiders because we are able to protect them. However, our partner is a single mother with a contentious ex. For that reason, we have all agreed we must remain very careful until her children are completely safe from outside interference. We cannot protect her children in the same way we can protect ours. We can best protect them by not being open in ways that would put her and her children at risk.