Generally all persons who immigrate through a family member must have a qualified I-864 filed on their behalf by the petitioner.  By completing the I-864, the petitioner/sponsor is assuring the government that the intending immigrant is either (1) financially self-sufficient and thus will not need public assistance, or (2) has family who will agree to financially support the immigrant at 125% of poverty level. 
Who May Submit the Affidavit of Support
A person who submits an affidavit of support is called the sponsor. In order to be a sponsor, one must:
Be a citizen or national of the US or a permanent resident;
Be at least 18 years of age; and
Live in the United States or a US territory or possession
Moreover, the petitioner must be a sponsor and submit an affidavit of support on behalf of the intending immigrant regardless of the sponsor's actual ability to support the intending immigrant.
Determining 125% of the Poverty Income Guidelines
A sponsor signing the affidavit of support must demonstrate that he/she earns enough income to support the immigrant and sponsor’s entire household at 125% above the federal poverty income guidelines.  To determine the size of the household that must be counted in calculating the 125% poverty level, the following persons must be included as part of the household:
Click here to read I-864 Affidavit of Support - Part II by Maryland Immigration Attorney Van Doan.
 Exceptions to the requirement include (1) self petitioning battered spouses and children, (2) where the petitioner has died after the petition has been approved e.g. widowers, (3) children are automatically derived US citizenship when they became lawful permanent presidents, and (4) persons not earned or inherited 40 quarters of qualifying income.
 Sponsors who are members of the Armed Forces only need to demonstrate the ability to support the intending immigrant at 100% of poverty level.
 The federal poverty guidelines are updated annually each spring. A current version of the Property Income Guidelines may be obtained at http://www.aspe.hhs.gov/poverty/index.shtml.
 This only applies to immigration on or after December 19, 1997.