Not exactly sure what to ask...

I have a friend who I would like to adopt as a sister. We both grew up in broken families and when we found eachother, we became the family that we had both been looking for since we were children. I've been searching online for quite some time now for information on this, but I don't think I'm searching the right terms. I realize that there are more important adoptions being helped here, but this is very important to us. Especially now that her husband and she just had a baby girl, and we would like to be a family in the eyes of the law. I thank you all in advance for any information you can provide me. Does anyone know the legal term I should be searching for? Or how much something like that should cost? I keep hearing that regular adoptions cost upwards ten thousand dollars... Thank you all for your time.