The “dizzy finger” means that the diseases are not carefully choosing who they pick.
This is an example of personification. Personification is a type of figurative language in which something that is not human is described and given human qualities. It is a way to help the reader picture and understand something in a creative way.
In this comparison, the disease is being compared to a person who is dizzy. When you are dizzy, you can’t point to something. Think about playing “Pin the Tail on the Donkey.” You know that game where you have a friend spin you around a few times, and then you have to pick where to pin the tail, and you win a prize if you pin your tail in the right spot? You may not actually have played this game, because it is kind of old-fashioned, but this might have been the image the author had in mind when she made up the personification. She might have had some other dizzy-picking metaphors in mind though!
Esperanza seems to be searching for an explanation for what happened to her aunt.
Maybe the sky didn't look the day she fell down. Maybe God was busy. It could be true she didn't dive right one day and hurt her spine. Or maybe the story that she fell very hard from a high step stool, like Totchy said, is true. (Born Bad)
She is young, and diseases like this are scary for her. She does not know quite what to think, so she looks for meaning. Since she seems to have a poetic mind, many of her images have a touch of the figurative to them. The more realistic explanations that she gets from others are less satisfying to her.
All of this is a lot for young Esperanza to take, especially with the guilt she feels. Like most young children, she internalizes what has happened to her aunt. Her mother is certainly not helping, telling her she was born on an “evil day.” She feels guilty when she and the other kids make fun of aunt, because they forget that what they are doing is wrong. It is just too “easy.”
It was a game, that's all. It was the game we played every afternoon ever since that day one of us invented it. I can't remember who-I think it was me. (Born Bad)
Of course, making fun of a sick person is a terrible thing to do. Esperanza is a child though, and children make mistakes. While she might have felt bad, Aunt Joan surely understood that the children were just scared and immature. Dealing with a dying person is grating. They were too young to handle it, and being asked to deal with something they were emotionally unprepared for. It does not mean they were born bad.