In the US, if the actors were related, then that would be illegal. The characters being related is totally immaterial, though. If it was relevant, then certain scenes in Game of Thrones would have been illegal. The law says nothing about fictional siblings getting it on, just real ones.
Payment processors, on the other hand, care a lot.
Legally it does make all the difference.
Depends on the country.
Only PIV sex is illegal for related people over here.
Anything else (anal, oral etc.) is a-OK for related people because there is no risk of children being conceived.
In the US, if the actors were related, then that would be illegal. The characters being related is totally immaterial, though. If it was relevant, then certain scenes in Game of Thrones would have been illegal. The law says nothing about fictional siblings getting it on, just real ones.
Payment processors, on the other hand, care a lot.