Katarina was just over eighteen months old when the family moved to Ainola in the autumn of 1904. However, judging by her diaries, which are kept in the National Archive, she was quite soon able to appreciate the lively interaction between the artistic families living round Lake Tuusula. Katarina considered fancy-dress parties the greatest fun, but in her diary she also writes of games involving aeroplanes, and other children’s pastimes.
Katarina tended to follow her own path, and her sisters considered her to be the daughter who was dearest and closest to their father. She was the best piano-player of the children and she even tried her hand at composition. Her father made a few corrections to her notes, at which point Katarina abandoned the work because it was no longer her own. In 1917 she moved to Helsinki to attend school there, but she had to come back to Ainola immediately after the battles of the Civil War broke out at the end of January 1918. She continued her studies in the autumn of 1919. Katarina took her matriculation examination in the spring of 1920 and she gained the highest marks in five tests of six. The following autumn, after she went to study the piano in Stuttgart, Sibelius felt that Ainola had become deserted
On 30th August 1924, when Katarina was 21, she married the lawyer Eero Ilves, aged 37. Eero became Sibelius’s confidant in financial matters, and he filled in Sibelius’s tax returns over the decades. The family kept in close touch with Ainola. Katarina was with her father when he passed away on 20th September 1957. After the composer’s death Katarina and her sisters took it in turns to come to Ainola so that their aged mother would never have to be alone. When Ainola was opened to the public as a museum in 1974, Katarina was one of the guides to the house.