Should incest be legal if they don't have kids?

Mary • My beliefs and opinions are constantly evolving
Had an interesting conversation with my friend just now 😅
So she believes incest should be legal as long as they don't have kids bc from what she said, it's not harming anyone else, they're two consenting adults, and that if they don't want to be in an incest relationship then they don't have to be. 
Btw, incest as in siblings in this situation, not cousins. She thinks it's weird and would never do anything like that, but she also thinks that two consenting adults should be able to get married. 

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