Ok, yeah I got that wrong.

Raised by and adopted by are two different things. You can raise someone and not have them considered as your child. Not to mention the whole definition of a step sibling is sharing one parent, adoption doesn’t make you a step sibling

No, not at all. Neither of them shared a parent. They’re technically cousins, but the amount of generations between Arwen’s uncle and Aragorn make the familial link not matter at all