>>80880>If you believe in "power dynamics" you can't mutually believe in consent at the same time.
Let me demonstrate. If a daughter decides to get into a relationship with her father (who raised her since she was a baby) once she turns 18, how likely is it that that relationship is going to be fair and equal when she has acted in the parent/child relationship power dynamic for so long? She is at considerable risk that her father will treat her in an unequal way because of this dynamic. Even if her father treated her lovingly in the relationship, the dynamic still remains, likely has unhealthy results, and can easily, easily be abused. Consider why 13 year olds in school having relationships with their teachers is considered unsafe, even if the teacher really does have an interest in the person they're in a relationship with.
With most siblings, it's not an equal relationship. Most sibling relationships have order instilled in them (elder vs. younger) and look at each other, depending on this order, usually. They also might have special roles assigned to each other from a young age. The point is, if they grew up with each other, the potential for inequality in the relationship, as well as abuse, is huge. Ask people who are molested by siblings - it's often not a clear cut "someone didn't want it". The sibling's relationship to each other was utilized for access that usually wouldn't be granted. The same would apply to a potential relationship once a person's 18.
I mean, consider how that would impact certain scenarios - you could groom relationship partners for yourself and legally nothing could stop you as long as long as they hit a certain age. Obviously, this isn't the main situation I'm thinking of in my above responses, but it's an opened up possibility.
I think of it similar to the legal age of consent isn't technically necessary in that I'm sure there have been positive relationships of 12 year olds with 20 year olds, etc. But, the age of consent provides a certain protection to its victims.