I just lost my last sibling I have the most amazing best friend. We already consider ourselves as sisters, would love to become legal siblings. How do we do this? All we have is each other as far as siblings go.

Well, which one of you wants to give up her parents?

In order for an adoption to happen, legal parentage needs to be severed, and the new parents adopt.

You cannot become legal siblings without one of you losing her parents legally.

Like Syn said, adoption forms a parent/child relationship, not a sibling relationship. If you were to adopt her, she would be your daughter.