Someone explain this Oriemo sh'it to me

You could probably also throw some Westermarck Effect in there to explain it.
But i doubt that the author thought of that when writing the story.
Then again, the characters were probably also too old for it to apply to them anyway.

I have always maintained that the Westermarck Effect is NOT a real thing. There are too many variables that make it a grey area or flat prove it wrong for it to be legitimate. If you aren't at least a little bit attracted to relatives of close age, then they are either not physically attractive or peaches; it goes by the same ruleset as a normal person. Taboo is all societal in any case, if it's never brought up then kids don't even know that it's wrong in the eyes of others.

at least from the people i ask IRL who also has a sister who watches this show, kirino is generally considered(in their opinion) one of the least likable characters in the show compared to the rest.

Kanako>=Saori>Ruri>>>pomegranate>>>more pomegranate>>>Kirino=Ayase

Manami and Kuroneko's sisters are not romantic interests in the series, but they are my favorite girls aside from Kanako.