见红字部分。
https://my.uscis.gov/helpcenter/article/how-do-i-financially-sponsor-someone-who-wants-to-immigrate
里面有个PDF，其中的一个问答如下：
I filed the I-130 immigrant petition for my relative,
but I do not meet the minimum income requirement. Can
anyone else be a financial sponsor?
If you do not meet the financial qualifications, the income of certain
other household members can be added to your income level if
they sign a contract on Form I-864A, Affidavit of Support Contract
Between Sponsor and Household Member, agreeing to make their
income or assets available for the support of the relative applying for
permanent residence.
If you still cannot meet the financial qualifications, another person
must complete a separate affidavit of support to become a joint
financial sponsor of the person's immigration. The joint sponsor
must meet all sponsorship requirements separately, including the
minimum income requirements or his or her household, and must be
willing to assume, along with you, financial liability for the sponsored
immigrant(s).
All sponsors must be U.S. citizens or permanent residents, be at least
18 years old, and be living in the United States (including territories
and possessions) when they file the affidavit of support.
