How much can we be sure, if this would be made legal, that the consent of the adult siblings would mean the same thing as the consent of two random unrelated people. If someone wants to have sex with their sibling who is not really happy about that idea, how much opportunity would they have to pressure them into “consent”, if they merely have to wait until their 18th birthday, as opposed to the opportunity to pressure into “consent” someone who is not a relative?
Even if the person who wants to have sex with their sibling waits with the coitus until the sibling’s 18th birthday, they still have plenty of opportunity to “groom” them before they are 18. Imagine siblings with large age difference, where the older sibling uses their mental superiority to “brainwash” their younger sibling, to ruin their other relationships and make them socially isolated, to make them emotionally dependent, so that when the younger sibling becomes 18, they are not in a position to say “no”.
Also, many people at 18 are not really ready to be economically independent on their family. Imagine a family with two children, one strongly loved by the parents, the other disliked by the parents. When the unloved child becomes 18, their sibling can blackmail them into sex by threatening that if they refuse, the popular child will convince the parents to throw the unpopular child out on the street. (Yes, blackmail is technically illegal, but you would have to prove it. Also, if you are the unpopular child, getting the popular child thrown in jail will not help you get your parents’ love; you will still remain on the street.)
In families there is too much power disparity: parents vs children, but also some children vs other. If we care about consent in sex, it is better to not leave open any paths that would allow abusing this power disparity to enforce “consent”. Making sex between close relatives illegal is a simple Schelling point.
(trigger warning: rape)
How much can we be sure, if this would be made legal, that the consent of the adult siblings would mean the same thing as the consent of two random unrelated people. If someone wants to have sex with their sibling who is not really happy about that idea, how much opportunity would they have to pressure them into “consent”, if they merely have to wait until their 18th birthday, as opposed to the opportunity to pressure into “consent” someone who is not a relative?
Even if the person who wants to have sex with their sibling waits with the coitus until the sibling’s 18th birthday, they still have plenty of opportunity to “groom” them before they are 18. Imagine siblings with large age difference, where the older sibling uses their mental superiority to “brainwash” their younger sibling, to ruin their other relationships and make them socially isolated, to make them emotionally dependent, so that when the younger sibling becomes 18, they are not in a position to say “no”.
Also, many people at 18 are not really ready to be economically independent on their family. Imagine a family with two children, one strongly loved by the parents, the other disliked by the parents. When the unloved child becomes 18, their sibling can blackmail them into sex by threatening that if they refuse, the popular child will convince the parents to throw the unpopular child out on the street. (Yes, blackmail is technically illegal, but you would have to prove it. Also, if you are the unpopular child, getting the popular child thrown in jail will not help you get your parents’ love; you will still remain on the street.)
In families there is too much power disparity: parents vs children, but also some children vs other. If we care about consent in sex, it is better to not leave open any paths that would allow abusing this power disparity to enforce “consent”. Making sex between close relatives illegal is a simple Schelling point.