I think the monk as bodyguard is a cool concept but believe the rogue offers more options. See below.
Your character could be the cavalier's (full) sister. Older??* Which gives you relations to the half-brothers and we have one big (slightly odd) family.
All kinds of RP possibility - The older sister (say six years older than all the boys) saw what dads indiscretions did to her mothers health. She then hates(and loves) her father but since he's not around she takes it out on the bastards. Who are totally innocent btw.
As to monk vs rogue? Both would be good but with rogue being the sneakier class and all it would be my choice.
- The unarmed/unarmored disguise need only apply in urban settings correct? Out exploring you can don your leathers and wear your crossbow openingly. "In town" you use your skills, feats, and talents to help you seem the "lady of court".
- sleight of hand for hiding weapons(dagger,sap, etc.)
- Catch Off Guard (for using candlestick holders as weapons)
- Major Magic to cast mage armor (You'll probably get bracers of armor before this though.) Or UMD and a wand or scrolls to help with your AC
- Lots of talents to help with both the intrigue of court and survival in the wilds (Canny Observer, Fast Getaway, Snap Shot, Surprise Attack)
- Feats too. I think if you go ranged over melee you could get a few combat feats usable in both rural and urban settings. Deadly Aim and then the Shot on the Run tree to get Parting Shot at level 8.
* Which brings me to age talk. How old are you going to make your wizard DT?
HM