Both sisters have been dealing with leg problems, and U.S. captain Mary Joe Fernandez has been holding open a spot for the matches at Birmingham, Ala.
The roster can be changed up to one hour before Friday's draw, but Venus was expected to tell Fernandez her status Tuesday. The matches are Saturday and Sunday.
Serena has been sidelined by a left knee injury since winning the Australian Open. Venus wore wraps on both legs in the tournament final on April 3 in Key Biscayne, Fla.
The rest of the team includes Melanie Oudin, Bethanie Mattek-Sands and Liezel Huber.