I mean Brooke really does always know when someone has LEFT HER WITH NO CHOICE.
Today, she confessed to their mutual sister that she bumped uglies with Katie's husband (oh sorry to be so crass, they would refer to it as "made love" or "what they shared" because what could be more beautiful), and she justified it over and over and over again by explaining that FIVE MINUTES BEFORE, Katie had said the marriage was over and she totally meant it. Donna suspects that Katie was just angry but, no, Brooke knows that Katie meant what she said. It hasn't occurred to her quite yet that EVEN IF SHE'S RIGHT AND KATIE WAS DONE, you actually do not get to fuck your sister's husband five minutes after she announces she wants out. You actually do not. Even if said married sister is super-hard on her husband by being all judgmental and not having energy for sex when she's taking care of a newborn and not wanting her hubby to get wasted and, like, knock nieces off of balconies or whatevs. THERE IS LITERALLY NO CIRCUMSTANCE UNDER WHICH THIS BEHAVIOR IS OKAY.
But Brooke doesn't get that, because she's "wired" wrong. (Lord do I hate it that Taylor's right!) Brooke is legitimately shocked and baffled that this is blowing up in her face.
(Does anyone else still get distracted after all these years in scenes between Katherine Kelley Lang and Jennifer Gareis by how much they totally really do look related?)
Elsehwere, Bill thinks he's just fantastic by re-committing to Katie despite all of the throwing himself at Brooke and the history of cheating. Oh and right, remember how he hates it when people walk out? Katie keeps walking out! HOW CAN SHE EXPECT HIM NOT TO SCREW HER BIG SISTER?!
And the show is just twisting the knife by having poor Katie constantly apologizing to Bill and Brooke about how horribly unfair she was, when she was completely right all along.