Weirdly, we don't get a terribly in-depth picture of Margot Frank. This might be because she's older and less exuberant, or because there's more than a touch of sibling rivalry going down in The Diary of Anne Frank.
Here's what we know: Margot Frank, Anne’s older sister, is sixteen at the start of the story and eighteen at the end. At least through Anne’s eyes, Margot is smarter, quieter, prettier, more grown-up, and more of a "proper lady" than Anne:
I know I’m starting at a very young age. Not even fifteen and already so independent – that’s a little hard for other people to understand. I’m pretty sure Margot would never kiss a boy unless there was some talk of an engagement or marriage. Neither Peter nor I have any such plans. I’m sure that Mother never touched a man before she met Father. What would my girlfriends or Jacque say if they knew I’d lain in Peter’s arms with my heart against his chest, my head on his shoulder and his head and face against mine! (4/17/1944.2)
The two sisters don’t often get along, are not close friends, and don’t confide in each other much.
Margot doesn’t get much attention in Anne’s diary, but we do know from a letter that Margot sends Anne that Margot wishes she had a friend to confide in. She envies Anne’s relationship with Peter (but only the idea of having such a friendship, not actually the boy himself).
Margot shows that she is mature and caring by encouraging Anne’s relationship with Peter, and by not being bitter with Anne for having a close friend. Whenever we get glimpses of Margot’s personality, we can’t help but think that she’s probably underappreciated by Anne.