In the divorce decree, it’s inferred. It said “the parties” meaning Ann and Lolo had a child who was no longer a minor and then Maya is listed also as theirs. Obama would have been referred to as a step-child. Not to mention, BHO was known as Barry Soetoro in Indonesia.
When a child is adopted, they are not legally step-Children. THe law sees them as children born of that parent. This is why Barry got Lolo’s citizenship and Nationality, it is an automatic part of the adoption process.