How can the kids (girls) on foster care be adopted?

I currently have a 13-month-old foster son, whom we got in January at 8 months old. He was neglected, and is failure to thrive. He has a half sister, and a sister. Both whom are in care of family. CYS doesn't know much about them but tell me that they were not taken by CY. Both sisters have the last names of people they are living with. The biological mom still states that she is trying to get the girls back. Any clue on how the family could have taken custody of the girls, and adopted them if she states they are trying to get the girls back? The reason I'm really concerned about it, is I just don't want something to fall through the system if aggravated circumstances could apply.