Researchers claim a relative formally and legally adopted his step-children.
Following the money is the best way to find out. The relative’s estate was settled in Illinois in 1904. He had a relatively significant amount of real and personal property at the time. The step-children were alive in 1904.
The relative does not name the adopted children in his will–even to give them a token amount so they cannot say they were forgotten. They are not listed in the “Order of heirship” where his biological children were listed. A reference to contemporary state statute indicated that legally adopted children could inherit from their adopted parents. That seems to suggest that the step-children were not legally adopted by their step-father.
Reading state statutes can be tedious, but it can often answer genealogical relationship questions.
Most 19th and 20th century state statute publications have been digitized and are available online through GoogleBooks or other sites containing digital images of books. The Advancing Genealogist contains links to images of revised state code organized by state.