They probably didn't pay much attention because they probably thought that that one of their children was going to die. It's difficult to divide your time with as many kids as they have equally, but when you through babies and a kid who has a brain tumor in the mix, the oldest, completely healthy child should be trusted to understand why she's not getting the same level of attention. Malignant or benign, a tumor is terrifying, especially one in an almost always inoperable place.
Also by this point, Kayla was well into her teens, and by most standards should be more interested in helping her brother rather than the attention she is missing out on because he is ill.
I remember parenting books having (might be outdated) chapters where they usually have 3-8 year olds being coached about what it will be like with another sibiling, and how if your parent pays more attention to the baby it doesn't mean they love you less...
It seems more Kayla who has an income is mad her parents aren't spending money on her, even though she has her own money...
I think another reason her parents were reluctant to fund her ventures was that she cited that she was quick to pick up and drop and interest. All the things she wanted take a lot of money and time on the parents' part.
The other brothers obviously have shown a commitment to something.
This is VERY important. Voice lessons could cost at least 100$ a session and that's only if you are good friends with the voice coach. Violins are extreemly expensive as well as lessons for them! And Kayla shows that her interests change easily over the years! (I am glad she didn't have horseback riding which is the most expensive hobby)
Perhaps if she wanted to try again and reestablish herself to her parents as someone they would want to invest money in, and not a wishy-washy complainer, she could go online for some type of lesson and learn herself. If she showed that she could produce something without needing financial backing from her parents, they would be more willing to support her once she gets to a certain level that the lessons online were too easy, or the level she is at is not online etc.
Kayla has said he is going to be a game developer. She wants an in into the video game industry and its sitting in the next room. He would probably love if his big sister stopped hating him for existing and getting sick and would spend a lot of time with her if she just tried.
This is very important. If she does this she could get experience+recommendations, she could also use this to produce a portfolio. But not only that, it could show her parents she is willing to try and that she is serious about something, and also IDK be a good sister...