Well, two things need to happen, you need to get the 'angel blanket' carved into your bones, and so will he whenever he's old enough. But, it should be obvious to her, even, that he will. I mean Dean's Micheal's and she was Castiel's meatsuit.
It's just the mojo, I think if your great great grand kids-with the demon so muddied that no one would know- got close it would have set off warning bells. That's just how that thing works.