The last Red Devil (Boones sibling) ended up killing Gigi during their Thanksgiving meal then went to go cover their alibi. We also found out that Boone + the other red devil are the siblings of Grace through Petes sloothing.
I think the other red devil might be either Hester or Chanel #5
Hester: She was "killed off" so no one would go looking for her which would give her a huge time frame where she could go see Gigi, kill her and then show up at Chad's house later in the evening which is when they first figured out that she wasn't really dead.
Chanel #5: She ran off and said she was going to go send Thanksgiving with her family but hers was one of the only ones we did not see. She left the night before then possibly could have gone to Gigi, killed her then made that gross 8 meat stuffing (Which is something only a psycho would make). It would also give her a flimsy alibi as she wasn't actually with her family. No one could vouch for where she was because it turned out that her family had gone away for vacation. Plus everyone things Chanel #5 is too dumb to kill people which is why no one in the house is really suspecting her.