My sister wasn't a brat, and I wasn't a bully.

What does bullying have to do with it?

However, I never tried to break down her self-respect.

I've never seen anyone advocate breaking down a woman's self-respect, so I'm not clear on the relevance here either.

How flexible is the "bratty little sister" model for coverinig situations where the sister is right?

Brothers and sisters can disagree, can they not? Sister isn't required to agree with brother, nor vice versa.

Think of it this way: right now, you appear to think that the problem is that if the guy pushes one way, then she has to go along with that.

Now, reverse the model: pretend that if she pushes one way, the guy has to go along with that.

That's the mental model most men (AFC's or Average Frustrated Chumps in PUA lingo) have about relationships.

By default, "nice guys" think they have to agree with everything a woman says. This is especially the case if the woman is attractive to them, and they really want her to like him.

You might not think this is most men's model... but that's because most men don't approach the women they're attracted to in the first place! And the ones that do, tend to get written off as unattractive or not relationship material, precisely because they're too eager to please, doing too much, "well, what do you want to do?", etc.

PUA appears biased the other way, because it's trying to train AFCs that they need to actually have an opinion of their own, and be able to maintain that opinion even when a woman they're positively infatuated with disagrees.

Unfortunately, availability bias on the part of women means that you are going to think men are already too far biased this way, because the majority of the ones who come and hit on you in the first place are towards the further end of the wimpy-nice-confident-aggressive-asshole spectrum. PUA training is aimed at moving people at the low end of that scale towards the middle, not the high end off the scale.