A substitute sponsor is a person who replaces the main sponsor / the visa petitioner if he or she dies during the sponsorship timeframe. A substitute sponsor, however, must be a relative of the intending immigrant. People who may qualify as a substitute sponsor in terms of the relationship are spouses, adult children, parents, mother-in-law or father-in-law, legal guardians, grandchildren, etc. They also have to meet other requirements like being a US citizen or permanent resident, meeting the financial requirements, and living in the United States.