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Mom said, Derek, it truly is a cliche. There is no line, no permanent change unless that's your expectation going in. Monogamous couples who are friends first and then expect intimacy to be forever often make the mistake of neglecting their friendship, then can't rebuild it if they break up. Open relationships at least acknowledge that intimacy can be a temporary thing, which can help preserve those friendships.
But incest is also different in that it needs to be kept secret, so maintaining the family relationship in public becomes critical.