I think for you your cousin is trying to get the financial evidence for her parents. Because may be she doesnt make good money probablly  less than as the minnimum annual income riquirement.For that you have to fill up form i-864 as affidavit of support form and you have to show your income, three years's tax file report  and your greencard's front and backside of photo copy and you have to sign the form. and send it to national visa center.