@Eclat123 I think it's more of her specific actions and her choice of words. Note that she is supposed to be a noble lady, but she suddenly hugged him and call him by his name (even though they're not supposed to be that close with her new body)
"Incest is wincest" is already a philosophy developed in a lot of people these days (Frozen and Oreimo made it even worse). By now the cousins part is the thing that affects me the least.