During the incident with the Xi Fa Xiang Gao Shiatsu, we see that in addition to forgetting everything about Ranma, Akane is unable to form or associate new memories of him. She's even unable to retain his name, thinking that her friends are talking about "Rama" and "Rumba" (in the translated version, obviously. I'm sure that the original version had different word-play.)
The thing is, we sort of see a more subtle version of similar behaviour later on, with Akane being unable to make mental connections on a certain subject: P-chan. The Mark of the Gods arc probably being the most severe example of this.
During the Breaking Point training, there's a fairly large period of time where Akane, Cologne, and Ryoga are all isolated in the same area, while Ranma's off dodging bees. It's possible that during this time-period, Cologne made an attempt to manipulate Akane into giving up on Ranma by talking Ryoga up, and while doing so mentioned Ryoga's curse, causing Akane to realise that Ryoga (who she's only really met twice at this point, during the first fight with Ranma and the Ice Skating competition,) is her pet.
I'd imagine that it would be fairly in-character for Cologne at this point to use a memory shiatsu point to erase Akane's memory of the conversation, and implant conditioning that prevents Akane from ever realising the truth. She could probably also condition Akane to believe, as an incontrovertible fact, that Ryoga is a nice, upstanding young man who'd never do anything perverted. Once that was done, Akane wouldn't be able to think of Ryoga as a pervert, even if she walked in on him in bed with both her sisters.