From here:

Laws banning incest between brothers and sisters in Germany could be scrapped after a government ethics committee said they were an unacceptable intrusion into the right to sexual self-determination.

“Criminal law is not the appropriate means to preserve a social taboo,” the German Ethics Council said in a statement. “The fundamental right of adult siblings to sexual self-determination is to be weighed more heavily than the abstract idea of protection of the family.”

The problem for the Anglican Church will be this: if the church redefines marriage to include same sex couples, what possible reason could it have to exclude opposite sex siblings in monogamous, committed, faithful relationships?