I don’t blame Brandi for much in this. A couple invited her into the marriage under the pretense that she would become a second wife to the husband. As soon as she gets there, the couple’s massively dysfunctional relationship involves her in their problems. It’s not her fault that they irresponsibly invited someone into their unhealthy dynamic. She could and probably should have backed out at that point, but likely already had feelings for Bernie, which clouded her judgment (I do believe he was talking to Brandi behind Paige’s back and getting closer to Brandi than he led on). It’s her prerogative to have feelings for whoever she wants. She wouldn’t have known at the time that Bernie was slimy.
From then on she received contradictory messages from Paige and Bernie. Paige was not clear and behaved in a volatile manner (go on a date! No, don’t go on a date! No kissing! Never mind, just don’t tell me about it!) Brandi’s mistakes can mostly be explained by lack of communication and dysfunction on Paige and Bernie’s part. Brandi should be thanking her lucky stars because those two have proven themselves to be unstable assholes who can’t get along with anyone.