I do Jack. The reason being that in my experience, incest can come about in cases where there has been abuse within a family which has not been dealt with and because of this, the appropriate boundaries and 'off buttons' do not apply... So while you may still have two older siblings making this decision, in my experience, it's rare that they are making such decisions from the same premise as someone else would.
Also, you have mothers of children who have been abused by their fathers and who still have those fathers in their lives... They are unable to get themselves out of a cycle of abuse (even if no longer occuring, the mindset is still there) which means they are unable to protect their own children and also themselves. Again, boundaries and a sense of right and wrong are all wobbly.
I am sure not all incest cases are like this, but a lot of what I see is and for that reason, I will never be on board for incest becoming more exceptable. I'd be happy for it to be more talked about though :c)