You wouldn't say you'd fuck her (if you were a normal healthy functioning adult)
I can explain this in detail, but someone already linked the wiki article explaining it. In short, IF you have a sibling, it changes things, you dont view them sexually. It extends to cousins you spend a lot of time with growing up as well.
I can explain this as well, but it basically boils down to this. I have two cousins, both girls, same age range, about 1.5 years apart, oldest is 10 years younger than me. I grew up with the older one, she treated me like a big brother, I view her as a little sister. Even after years of barely speaking as we got older and she became a teenage girl and eventually a woman, and seeing her now in her 20s? Nope, thought of fucking her makes me sick to my stomach. On the other hand, the younger cousin (different sides of the family mind you), I didnt even meet until I was almost 27 and she was 16 getting close to 17. Instantly wanted to bang her. Hell after she turned 18 and we were talking a bunch, dunno we just became fairly close pretty fast, she ended up sending me underwear pics of herself. She was subtle about it, but considering the first picture was supposed to be showing off her belly button ring or something, and "incidentally" had her pants unbuttoned and open just enough to see her panties? yeah, she was baiting me. I fell for it.
To this day I have to remind myself shes my second cousin, because I didnt grow up with her and dont think of her as family in the same way I think of the other cousin as family because we grew up treating each other like siblings.
Naturally no one knows this.. she might because of the whole underwear modeling bit.. but no one else does. But my point is, even with cousins, if you grow up with them, it changes a whole fucking lot.
So no, you cannot objectively say you'd fuck your sister "if" you had a sister. Because HAVING a sister you grow up with VASTLY changes your outlook. But I dont expect you to understand this. Its something you legtimately have to experience to understand.