Anne says Margot has become quite sweet these days and isn't as catty. Why do you think this is so?

On Wednesday, January 12, 1944, Anne writes in her diary:

Margot's gotten much nicer. She seems a lot different than she used to be. She's not nearly as catty these days and is becoming a real friend. She no longer thinks of me as a little kid who doesn't count.

Although Anne grew up admiring her sister, that admiration turned into resentment and jealousy as the two aged. Margot seemed to be better loved by most adults due to her calm and non-reactive personality. Anne, in fact, used to sarcastically refer to her as "a paragon of virtue." When trouble was afoot, Anne was usually blamed for it first since no one might suspect that Margot would misbehave. 

Anne and Margot's renewed sense of friendship could come from necessity (as it was better for the sanity of everyone sharing this small space if the two would get along) or from Anne's budding love for Peter (which seemed to have a mellowing effect on the emotionally tempestuous girl). It could also be due to the fact that Anne seems to be maturing and gaining self-awareness, which is demonstrated in an entry shortly before this one in which Anne re-reads portions of her diary and scolds herself for her nasty words against her mother. We can see that Anne is growing up over the course of the diary, and as she transitions from being a moody little girl to a more thoughtful and sensible young woman, her relationships seem to transition as well.