First of all the only westerners who make this accusation are those who see our original Anime or who see our Anime which are not translated into any western languages, in other words westerners are basicly cencoring our Anime that are shown to other westerners that in the end can't see the full quality of our Anime. But there are a few case of fan translations who try stick to the original as much as possible who translate the conversations of our Anime characters with the same meaning as in western languages.
Now as for the reason why the westerners who hate our Anime accuse our Anime as incest stuff is because we Asians have different culture than those westerners and the culture which cause this misunderstanding is the culture of "how a couple refer to each other". Just like in our Anime which those westerners call as incest stuff, in our Asian culture the way our Asian girls refer or call their boyfriend is also as "big/older brother" while our Asian guys also call their girlfriend as "little sister". This is not just in one of our Asian countries such as Japan but also in our other Asian countries, the only difference is the term used which depend on which of our Asian languages are used. For more details about this including the reasons why we Asians are using those term then just go to http://antiwesterncosplayers.blogspot.com/2013/12/it-is-also-culturally-wrong-for-racist_4729.html
So this is really not because of the couple in our Anime are real siblings or not because of the couple in our Anime want to have a sibling-like incest love relationship, just a difference between our Asian cultures and those westerners cultures.