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.