I would say it really depends on the way it's carried out and on how deep you have gone into disturbing world of yaoi...
Like in "Not Equal" by Ike Reibun (Supernatural twist) most of the story they have a rather stranger-like relationship, so it wasn't as disturbing as I thought it would be.
But to read Ogawa Chise's "Kasatta Rasen" or "Ienai Itami o Daite Iru"(only stepbrothers) you must be into twisted stuff.
Even so the Japanese always love to prove how much they can simply crush our little world of cute and fluffiness ╮( ̄▽ ̄)╭ (so you're not the only one)
This manga was really cute!! but i have a question... Am i the only one that's disturbed everytime there's incest in a manga?