The split personality could be interesting. I would advise against something like bipolar, because it's so hard to RP accurately...and, also, psychiatry is not very popular in this setting, so people just will not get it! Plus, if she was really mean to the kids, Redly would not like her very much! If her "being mean" was generally good-natured and the typical sibling stuff, that would be acceptable!
I say the split personality could be interesting because it sort of feeds -- backwards -- into one of my ideas.
Let's say your character had a twin brother. Ma & Pa Treyburn were horrified, further condemning her and causing them to distance themselves yet further from the kids. Twins Are Bad. Mmkay? For simplicity, let's call your characters John and Jane as I explain this.
John and Jane grow up the target of their parents' disdain. Everything, somehow, is always their fault...especially when it's not. And then something Bad happens to John. He dies. (Added twist: Ma & Pa Treyburn are involved!) Everyone is sad, except Ma & Pa, because they're terrible people.
Jane tries to make up for the loss of her brother in the only way she knows how -- she tries to become him. Jane herself was just the sweetest little thing; John was a bully and a bit of a nightmare. Jane, when she chooses to be Jane, continues to be wonderful -- but when she chooses to be John, she over-acts, as children are wont to do.
So it's not completely split personalities... She's well-aware that she's Jane...but it makes her feel better to pretend that John is still there. It's not entirely normal or healthy, but it's something she could, as an adult, have recovered from -- or not, because it's up to you!
Now, we get to the part where we break the law. Jane has grown up seeing both worlds: that of a pretty, noble girl...and that of a rough and tumble boy. Redly, and, I assume Braxton as well, were not gentle kids -- they liked to get dirty, to fight, and to have a good time. If that appealed to Jane's personality, she may have decided to break the law to maintain that lifestyle. If her parents tried to marry her off to some idiotic jerk-face to further their own desires for power and wealth, she could have boycotted and run to Redly for help. Redly would have been (probably) a minor officer of some sort in the military by that point...and, really, what better place for her to hide than in the military as Redly's younger brother John? This could have gone on for some time, with Redly covering for her and her parents thinking she had further disgraced them and run off...
So, let's summarize:
*We have the potential for her to have developed somewhat extreme personalities. Their exact mechanic is, of course, up to you.
*We have a crime committed. Cross-dressing in Connlaoth is very much illegal. Depending on how big of a bounty you want on her head, additional crimes can be tacked onto this for maximum naughtiness.
*We have Redly helping her commit said crime, while simultaneously pissing off his parents big-time -- but also not putting a bounty on his head too. That's important!
You're free to shoot down any part of this (or the whole thing!) of course. I, er, got carried away.
