You should really care that the German Ethics Council (a government committee) has pronounced that Germany’s laws banning consensual incest between adult brothers and sisters ought to be abolished.
Now, it’s not because Germany’s laws directly affect the United States; they don’t, of course. And even within Germany itself, the ruling party of Chancellor Angela Merkel has indicated that it doesn’t plan on following the council’s recommendation.
The reason why Americans — and especially American liberals — should care about the council’s ruling is that it gives us a glimpse of America’s future.
The German council’s position is based on the claim that “the fundamental right of adult siblings to sexual self-determination” overrides all other moral considerations, including “the abstract idea of protection of the family.” That is very similar to the rationales that have been used to uphold reproductive rights and to strike down bans on same-sex marriage throughout the United States. And we have every reason to think the same logic will eventually apply to bans on sibling incest in this country.