It has grown clear to me that Abigail Breslin, Chanel #5, is the other twin/Red Devil.
During this Thanksgiving episode, we saw most of the major characters at Kappa House. Chad, Chanel, and Hester were all shown celebrating at the Radwells'. Gigi ended up dead, but her murder had to have occurred earlier in the day. The only unexpected character to show up at the Kappa House Thanksgiving dinner whom we didn't see during the course of the day was Channel #5, who claimed her parents were vacationing in the Maldives. My money is on her. That means she is Grace's half sister.