To Franziska von Karma Fans; What do you think happened to Franziska and Edgeworth's relationship to twist it into what it is today?

To be more specific, I want to call attention to Season 2 Episode 6 of the anime, in which it shows life in the von Karma household 3 years after DL-6.

In it, Franziska is, of course, unbelievably adorable, but one particular tooth-achingly sweet thing about the episode is that Franziska spends the entire day doing anything she can think of towards one end; making Edgeworth smile.

The episode starts off with her explaining that she dressed up Miles's pomeranian in a von Karma outfit because "you never smile." And at the end, when von Karma places the signature Cravaat on Edgeworth's neck for the first time, he smiles, and Franziska watches from the shadows with a content grin on her face.

The way she's in the shade like that obviously sadly foreshadows the feelings she would express later in life about having always been in Edgeworth's shadow, but it suggests that, for this time being, she was content to be in the shade if it meant Miles was happy. So what I've always wondered is, what caused this beautiful relationship, where the younger sister wanted nothing more than to see her older brother happy, to turn so sour?

The basic explanation is that von Karma raised them to be overly competitive and/or treated Miles with more respect than her, causing her to believe she had to supersede Miles to be taken seriously, but such a simplistic answer doesn't feel like enough to me.

Does anyone with more elegant headcanons than me have any explanation for this change, for why Franziska seems to downright hate Edgeworth in Justice for All?