If you filed an immigrant visa petition for your relative, you must be the sponsor. You must also be at least 18 years old and a U.S. citizen or a permanent resident. You must have a domicile in the United States or a territory or possession of the United States. Usually, this requirement means you must actually live in the United States, or a territory or possession, in order to be a sponsor. If you live abroad, you may still be eligible to be a sponsor if you can show that your residence abroad is temporary, and that you still have your domicile in the United States.
Section 213A of the INA permits both a "joint sponsor" and a "substitute sponsor" in certain cases.