Green Card Processing for Adopted Sibling

[Seattlegooner]

Firstly,I appreciate all the advise that you guys are providing in these forums. It is very helpful.

My sister is an american citizen ( Received her citizenship a year ago). She was adopted by my parents in the year - 2000 and now wants to sponsor my green card application. We do have the adoption certificate which has details about the adoption.

Is it allowed for my adopted sibling to sponsor my green card? Any advice about the processing would be much appreciated!