You are absolutely right, I did write that but you don't have to be bullish when you resist unreasonable demands, you can make concessions, you can stand firm without being confrontational.
She may have the children but they have 2 parents and both are able to make decisions or, more helpful, both make them together.
Winning DIL over is, of course, the best way to go but if you can't, you may just have to bite the bullet and say, "I'm sorry, but as much as I love you, I can't agree with you and my conscience dictates that I must treat all of my grandchildren equally."
The wrong choices do have consequences, I am thinking of the consequences to an innocent child who doesn't have a father by the sounds of it, and is now going to be without a gran.