I fully understand. The reality is that this is a difficult case to make -- because as long as there is an "intact family" and there is nothing odd, abusive, illegal or some form of mental illness that would make the biological parent
and a new partner unfit, the state gives them the same rights as two biological parents would have.
Essentially, a grandparent is in the same position here, as grandparent would be if both their biological child and a spouse were estranged from them. Ultimately, the parents, when there is no dysfunction among the parent and partner, have the final say.
A lawyer could be helpful identifying possible means to show that the parents are not fit. But that, of course, is a very aggressive strategy -- and is not one to enter unless the claims are real, unless one is willing to live the consequences of ill will.
Ultimately, diplomacy is a better tool than the law -- but that does not mean the law is without teeth. Its just that its muzzled until there is some extreme situation when it can be released. If there are any neutral third parties that both sides trust, perhaps they can help (religious or civic leaders are often helpful). Likewise, mediation may help -- but of course, both sides need to agree to have it.
I hope this explains; ultimately, this is a personal and emotional question. If one or two hours worth of fees to a lawyer to evaluate the case would give you peace of mind that you've explored all legal avenues, then its worth the money. But understand that the odds are long unless there is, as noted, such dysfunction in the children's home as
abuse and or neglect, or dangerous mental or social illness.
Sorry that I can't provide a more upbeat assessment, but I suspect you wanted the unvarnished truth.
I understand how painful it is. I'd be devastated in the situation you describe.
I wish you all the best as you work to resolve this matter.