https://news.ycombinator.com/item?id=5819935
TL;DR: This rant identifies a major cause: math is taught as if one should magically be able to comprehend its language and syntax without explanation.
Also in mathematical notation there is no concrete flow to how you evaluate an equation. You can start just about anywhere. With programming, the order in which things get done might be arbitrary (and frustrating at first) it is at least consistent.
Mathematical notation is really good, for some people, to describe, communicate, and explore relations. They can directly see that by increasing the value of the denominator the resulting value decreases. But without a tool designed by Bret Victor, my brain does not do that. I want to follow along a set of instructions, maybe iterate through a series of inputs, and then I understand how the operations applied to the input produce an output.
Math has always seemed to me to be badly formatted code.
That said, I agree that math is terribly taught. My mother and my wife both hate math, and talking it out with both of them, I've been able to pinpoint it to a single bad/discouraging (and sometimes straight out mean) math teacher for both of them. Once the teacher is mean or otherwise bad about teaching, they have not only failed as teachers, but have sometimes caused the students to put up mental walls that can be hard to tear down.
I suspect the sister mentioned in the OP may have had such an experience.