To adopt my sibling's children if am USC?

Are these children in the U.S. or a foreign country? I know you can adopt but if they are citizens of another country then you have another series of steps to go through. you might wait to seek legal council, you will most likely receive a lot of speculation here.

Immigration benefits for a non-orphan adopted child are provided under Section 101 ( B ) (1) (E) of the INA. The following condition must be met for a non-orphan child to be eligible for an immigrant visa.

The child must have been adopted under the age of 16 (or is the natural sibling of such child who was adopted by the same parents while under the age of 18). The adoption must both be legal and final. Merely raising the child since birth or taking in a child for humanitarian purposes does not constitute an adoption. For immigration purposes, the adoption must create a legally-binding parent-child relationship or confer upon the child the same rights as a child born to the adoptive parents (i.e. inheritance).
The child must have been in legal custody of the adopting parent(s) for two years. “Legal custody” means the assumption of responsibility by an adult over a minor under the law of the state and under the order or approval of a court of law or an appropriate government entity. This means that a legal process involving the courts or a recognized government entity take place to award custody of the child to the parents. The date an adopting parent is granted legal custody of the child may be counted toward fulfilling the two-year legal custody requirement. Otherwise, the date the adoption is finalized shall be considered the start of legal custody. An informal custodial or guardianship document, such as a sworn affidavit signed before a notary public, is insufficient for this purpose.
The child must have resided with the adoptive parent(s) for at least two years. Evidence must be presented to establish that the adopted child and the adoptive parent(s) resided together in a parent-child relationship. Documentation must be submitted to establish that the adoptive parent(s) exercised primary parental control over the child, particularly after legal custody is granted and even during periods while residing apart from each other.