the reports I've seen aren't specific as to the legal (as opposed to emotional) relationship between Pitt and Angelina's children. (I refer to adopted kids or foster kids of my own even though there is no legal relationship) If he actually did adopt her kids,I suspect Brad Pitt and Angelina Jolie live in one of the several states which permits adoption by unrelated individuals - specifically same gender, although I suspect it would also extend to mixed gender:
>>Currently, second parent adoptions have been approved by appellate courts in the following states: California, the District of Columbia, Illinois, Indiana, Massachusetts, Pennsylvania, New York, New Jersey, and Vermont. In addition, there are statutes expressly permitting second parent adoptions in California, Connecticut, and Vermont.
Second parent adoptions have also been granted by trial court judges in: Alabama (selected counties); Alaska (Juneau); Delaware (selected counties); Hawaii (selected counties); Indiana (White County); Iowa (selected counties); Louisiana (Orleans Parish); Maryland (selected counties); Minnesota (Aitkin & Hennepin Counties); Nevada (selected counties); New Mexico (selected counties); Oregon (Multnomah County); Rhode Island (selected counties); Texas (Austin, San Antonio, & some other counties); Washington (King, Spokane, Thurston, & some other counties).<<
http://www.nclrights.org/publications/2ndparentadoptions.htm