Nephew or not has nothing to do with. You told her you are principled, you are not. Whether I'm better than you or not has no bearing on her decision. It's coming and the fact you don't know why makes it all the more justifiable in her eyes. The fact you argue against it and implore her not to do so only strengthens her resolve in the matter.
I have my list of things to work on, but none on my list are of the "non-negotiable" variety in her eyes. "Real men don't welsh" wasn't just a catch phrase, it's her golden egg.