It's not like they can have kids, and they didn't grow up together either, so there isn't really a problem.
That aside, hooray! New Kuwabara! I hope this is just as great as Tankobu!
Actually, there is one : Society.
No matter how you can accept it, and beyond the genetical risks, peoples are arsholes... If they hear about it. and thrust me, they will, they always do. it's not a matter of "if", only "when", You're life becomes hell. for this reason alone, Incest IS a problem. because it means you always have to face someone's judgement. Like homosexuality, but worse, since the chance for you to find a soul comprehensive or tolerating you are far lesser.