Do I have to file 2 affidavit forms to petition for my sister?

Filing one or two affidavits of support depends on the income and family composition of the sponsors. If you have no children, and your husband makes more than $25,200 a year (add $5200 for each child or adult dependent in the family), he can co-sign your affidavit of support. If your husband's income is less than the required amount, you will need a second sponsor (and a second I-864). But that seems to be the least of your problems. It takes over 12 years for a brother or sister of U.S. citizen to receive an immigrant visa. Filing of a petition by her brother or sister does not give the future immigrant the right to stay in the U.S. If the future immigrant stays in the U.S. after her status expired, she will become ineligible for a green card. Worse, there is a penalty for overstaying: if the alien overstays for more than 180 days, she must live in her own country for 3 years before becoming eligible for a green card (or any visa); an overstay of 1 year or longer entails a banishment for 10 years. Your statement that your sister is "doing her PhD and working at the university" while "her visa expired few months ago" is troubling for several reasons. If your sister came here to study, she should have been on an F-1 visa. F-1 status does not expire - so long as the alien remains enrolled as a full-time student and does not violate the terms and conditions of her visa. One of such conditions is that the alien cannot accept employment without special permission of USCIS. If the student starts working without USCIS authorization, her status terminates automatically, and the overstay count commences. Now, your sister should clarify what her immigration status is at the moment. If her status expired and she is in overstay, or if she is working without authorization, she should talk to an immigration attorney ASAP.