Even though contraception is widespread in this day and age, if large number of siblings have sex with each other, some of them will inevitably end up having kids.
A second problem is that the energy and emotions and time they devote to their incestuous relationship isn't going to a relationship where they might have kids.
5 years ago, I would have thought logically, and said that if they don't want kids and have access to effective contraception then it isn't a problem. But now I would think probabilistically, and say that even if they are 99% sure they don't want kids, they are about 50% likely (assuming standard levels of overconfidance) to change their minds around 30, and now they are really heavily invested in a relationship which cannot lead to healthy kids, and the sister's biological clock is running out of time.
So, it's certainly a bad idea, although that doesn't automatically mean it should be made illegal, depending upon whether you believe citizens should have the right to make bad decisions.
The same reasoning would suggest that bisexuals should only get into same-sex relationships. Would you say that as well?
I disagree with the idea that they can't have kids. They can adopt. The girl can go to a sperm bank.
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