#4 The Mysterious Visitor by Julie Campbell, 1954.
Now that school has started again, Trixie and Honey see more of the other kids who live in Sleepyside, including a girl named Diana Lynch. Trixie has known Diana for a long time, but ever since her father made a lot of money, Diana has been distant.
Di is deeply unhappy because her parents, particularly her mother, have been trying to live up to their new status by having the household run by fussy servants who don’t care much for children. Diana always has to wear formal clothes, she doesn’t get to play with her younger siblings as much, and she finds it increasingly hard to just relax and have fun with friends.
When Honey and Trixie invite Diana to become a member of the Bob-Whites, she tells them about her long-lost uncle who has recently come to visit her family. Her mother is happy about finally meeting the brother she has not seen since she was a baby, but Uncle Monty has been making Di’s life miserable. Whenever she makes plans to have a Halloween party or do things with friends, he tries his best to interfere and ruin everything. Uncle Monty is rude and offends people. Trixie is convinced that Di’s Uncle Monty is not what he seems to be, but she’s not sure how to prove it.
I love the description of the Halloween party that the kids have at Diana’s house!