Family adoptions are virtually impossible to count for immigration purposes.
1st of all, what country are they citizens of? What are the adoption laws like there?
In order to adopt a child and bring them to the U.S., the adoption has to meet one of three possibilities under U.S. immigration law.
Ordinary adopted children (non-orphans), (this was created for people who were immigrating and had adopted children) have to meet INA 101(b)(1)(E). This requires actually living with the child for 2 years BEFORE you can immigrate them. However, the petitioner may be either an LPR or USC. These use an I-130 or accompany a parent on an I-140 (or even can be petitioned via an I-360 or I-730). This one does not require a home study for USCIS purposes but that is something that could still be required for the adoption anyway.
Two varieties of orphans exist in immigration law but the petitioner must be a USC.
INA 101(b)(1)(F) defines non-Hague orphans immigrated via form I-600 and may be preceded by an I-600A.
INA 101(b)(1)(G) defines Hague orphans immigrated via form I-800 and may be preceded by an I-800A.
These last two require the home study etc..
Your late brother's children are orphans and your USC spouse could proceed that way.
See a lawyer. Affordable help might be found at:
http://www.justice.gov/eoir/statspub/raroster.htm
Most EOIR pro bono lawyers tend to concentrate on asylum cases but you could try them also at:
http://www.justice.gov/eoir/probono/states.htm