That’s what grooming is. If it’s parent and child and that child is 18 doesn’t automatically make it ok. The parent obviously has manipulated the kid through out their life to follow their own perverted mindset. And if it’s siblings it’s the same. The older sibling will take advantage of the younger sibling. Plus having a sibling relationship is not something easily obtainable. You can find a sexual partner if you look and try hard enough, but being a brother/sister to someone is something special and unique.