While Piper is seeing her past, and Phoebe is holding Paige crying after she stabbed her, Barbas appears corporeal for both of them. They don't seem to be seeing him, only hearing what he's saying. However, when he's about to stab Phoebe, Piper comes and blows him up. So she sees him, and if she could see him, surely Phoebe could as well. In that case, how can they allow themselves to listen to him and let him manipulate them since they know he's tricking them?