I am a US citizen and my husband is a perm. resident. Before coming to the US he raised his brother's son, after the death of both the child's parents. The child is now 10 years old. He thinks his mother may have actually filed adoption papers and legally adopted the child, making him my husband's brother. Am I, as the US Citizen spouse, eligible to file the I-130 for this child?

I don't see how whould that be done, so I say - NO.

USC can sponsor spouse and spouse's child as dependant, own children, own parents, own siblings.

PR can sponsor only heis/her spouse and children.

No.

If your spouse adopts the child (or you, it is even better) then it is possible.

Also, the difference if you, the USC files the I-130 in terms of waiting time is a lot shorter than if your husband does is as a LPR. Permanent Resident filing I-130 waiting time may be at least 5-6yrs.

Isn't there some rule about adopting the child before...I don't know. I mean, if i adopted the child now, would he be eligible for me to file?

If the child is now your husband's brother, legally, he could petition him, while he is still an LPR, and upgrade the petition once he becomes a US citizen himself. Still it would only then be a family fourth preference petition.