It's true Brittany's only successful move in the game was to get Devin to veto her. She hasn't made any effort in the game to make an alliance or secure anyone's trust.
I think Amber is correct though that the fact she's always the one to be a pawn is a signal that she's not quite so central to the alliance. She's on the right track talking to Nicole and Christine about fighting back.
Jacosta seems like she's going to have an anneurism. Every time there's any kind of pressure on her she gets sick or goes berserk. And I don't think it has anything to do with her being religious. She just can't handle stress.