I'm guessing she's about 5ft8 about 8 and a half stone and very active with workouts and stuff throughout the week.
So yeah she's probably at the very bottom of the healthy BMI for her height, and maybe if stressed or something looses weight.
But her complexion, her hair, all healthy. So she isn't starving herself to maintain this, also look at her muscle tone. Even recently in Boston in the green dress her legs, calfs all good muscle tone on it.
She's tall and slim, has family genetics that make her this tall slim build, her mother and sister are built the same, but they probably do not have the same resources as Catherine even though Pippa is a wealthy woman also, and they also don't have the same pressures to "maintain" as Catherine. So yeah maybe she does exercise more than her mother and sister.
She looks lovely. She's just a tall very slender, but athletic woman.