Yes. Your stepchild is a child for immigration purposes if the marriage that creates the stepparent-stepchild relationship took place before the child turns 18. However, where the marriage creating the stepchild relationship eneded by death, divorce or legal separation, you must prove that the relationship continues to exist.
Example: Juana, a lawful permanent resident, marries George who has has a 10-year-old
daughter, Soledad. Can Juana petition Soledad as her child?
Answer: Yes. Since George and Juana married before Soledad reached the age of 18, Soledad is Gina’s child for immigration purposes. Soledad became Juana’s stepchild as of the date of the marriage.