Ok, I need your help with a book title and/or author. When I was younger (much younger!) I used to read a book by a Swedish author (this much I do know), set in the early 20th century. There is a girl and a boy (sister and brother) with a father who is somewhat emotionally detached and reads books by 'depressing' authors. Anyway, the family gets a new nanny or maid or something and she turns out to be their half sister. The boy's name might be Roald and the 'maid's name might be Catherine. She's a bit of a wild one. In another book they (all three, I think) go away to work at an estate or something (can you tell my memories of these books are a bit vague) and there's a bit with a train ride. And it turns out that the people they work (?) for are also somehow related to them.
...I am hoping that maybe some of you might know what book(s) or author I'm talking about. Her first name might be Gunilla and I am pretty sure that she's Swedish - the books are set in Sweden as well. Help me, internets, you're my only hope!