I really underestimated how popular the toothbrush scene is
. He is a huge siscon but is still a great big brother, the only ones he fell for AFAIR are Hanekawa, who is pretty much his best friend now, and Senjougahara, his current and first girlfriend. Plus his two sisters have boyfriends are well.
Even if he were into incest, how would it be a minus in this competition anyway? This is a competition to choose the best husbando, morals don't matter here. He is a much better husbando than Laxus.
Well the reason is as soon as some people even see something that potentially spells incest, justified or not, alarm bells ring loud inside their heads. It's kind of a dumb thing. Not that I want to defend incest, but I always found it amusing how killing lots of people is okay (in the anime/manga world) but goddare you touch one of your closer relatives in a possibly naughty way and you're basically considered a freak or an asshole forever
Most of the time it's just for fanservice or comedy, because the japanese are into weird stuff to some extent (and we do love them for that, even if we can't relate to some things).
Same goes for other potentially icky stuff, like being into lolis. I actually find it rather... strange to be into, real or not, but I can accept it without judging people who like it, be they anime/manga characters or normal people. Everyone has his weird little fetish, so pointing fingers at others because they like what you find disgusting is just stupid. I'm pretty sure 99% of them wouldn't bang their own sister or a 5 years old kid, but in a fantasy lolis or brocons/siscons might entertain them or turn them on. I don't really care, as long as no real people are hurt. For some people accepting something like that is impossible though, which is a shame in my eyes.
Anyways, this got rather long xD In the end, Araragi is a siscon, a lolicon, he loves big breasts, he loves fluffy hair, he loves being dominated, yet he loves to dominate too, etc etc. In the show Araragi is basically into almost everything, for the sake of parody/comedy, but also for fanservice obviously. I never really thought of that as something making Araragi a bad person, because when the story starts getting serious, it always shows he's faithful and cares deeply about the people around him. People might think of him as being weird, but I have yet to see someone who doesn't find him likeable.