I admit I have a squick response to light sider incest--I think it's me twitching and saying, 'God, [light sider person] would never do that!' I don't *want* to believe it possible of characters I think of as 'good'. However, I can totally believe it of the Malfoys and the Blacks. (*have written Draco/Narcissa*)

It's weird, but I do have a strange 'levels of badness' system in my head--I find parent/child incest more squicky than sibling, and I find mother/child a bit more squicky than father/child. I'm not sure why... well, I have an idea, but it would take a while to fully explore.