Immigration Law Questions? Ask an Immigration Lawyer.
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The term “immediate relative(s)” is used to define certain immigrant relatives of U.S. citizens. Immediate relatives include:
I suggest a step sibling is not considered an immediate relative and you are not then legally obligated to disclose. But if you feel it will help you can mention him.
If the half sibling was helping to bring you to the US, then he would need to disclose the information of course and certain documents are required. But if the half sibling is not going to be involved in your immigration process, I suggest you are not legally obligated to disclose.