>>1418212
>Absent physical or psychological trauma in childhood (or physical separation from birth or a young age)
That's highly contextual. The Westermarck effect is mostly based on imprinting at ages 6 and below, and is going to be less strong if the exposure is only very partial. So the older siblings may experience little or no repulsion if the younger sibling was born 5 or 6 years later.
>Repulsion is guaranteed
Yes, to a strong degree, but it's not like it's something necessarily impossible to overcome, and it will vary from person to person. Evolutionarily incest is only strongly deterred, but not outright prevented, because it's still an evolutionary advantage to be able to do it in extreme situations where other mates aren't an option, since real genetic issues don't occur without multi-generational incest. It can be difficult to feel anything at all sexually at first, but once physical stimulation starts happening, that barrier weakens.
>unless it's a situation like that depicted in the game.
The situation in game is no excuse whatsoever for a lack of repulsion. Andy and Leyley were born two years apart and have always been attached at the hip, but the only hurdle towards incest has been socially conditioned reluctance on Andrew's part, not an inability to see Leyley sexually or get it up to her body. There's been no sign of the Westermarck effect in the game at all.
>Inb4 that shots and such scene where he goes flaccid after she brings up sibling talk
That was talk, reminding him of social pressures, not a repulsion to her body itself.