Incest isn't just about genetics. People have adult godparents, foster children, adopted siblings and a whole load of other relations without genetic connections being involved. There are power dynamics and a whole load of other reasons it isn't right.
All very fair points; I was thinking (but didn't specify) of the step-brother/sister relationship as mentioned in other comments.
And just to be clear, I'm not an especial advocate here! I'm just interested as always in establishing clear logical rules that can then be applied fairly.)
