That's one difficult thing, it can be pretty hard to tell from the outside whether it's the product of grooming or not. The same goes for a lot of very legal types of relationships though, so I don't think the possibility of it happening is a reason to completely criminalize it. The difference compared to the other things listed (children and animals) is those things can't consent, it's an impossibility.

I think enforcing some arbitrary age gap maximum for siblings though would make sense – incest between parents and children should be illegal full stop imo, and it's hard to believe that any relationships between siblings who are 10 years apart isn't from grooming.

That being said, I'm not sure that with our current shitty justice, law, and health system (in the US) that it's worth it to start giving equality to those types of relationships considering we just don't have the infrastructure or society to effectively prevent the legality being used to facilitate grooming. Society is too corrupt to prevent or bring justice for abuse at the scale needed. But people made similar arguments for incest being illegal as for interracial relationships being illegal so maybe I'm wrong.