no, to be honest...brother and sister is kinda, well "normal" when it comes to incest. You grow up with them, you have the same hormones, you're curious, and shit just happens.
Aunt and nephew is just all kinds of fucked up (or in the reverse direction). Obviously, this situation in GoT world is different, but since we are just doing a general comparison, you are typically comparing authority vs subservience. parent vs child. Aunt fucking her nephew, in general, is no different than a Mom fucking her son. You're commonly dealing with different generations with this issue.
again, this is obviously a different and unique situation, but when you argue on general terms, there really is no comparison.