When people talk about the gender wage gap they're not usually talking about the overall average, but rather doing like for like comparisons.
It feels like you're suggesting your sister chose to give up sales for fiction because there were other factors at play such as quality of life, that that is a decision men would be less likely to make, save therefore her actions have tipped the scale towards "women earn less."
But when people are talking about the gender pay gap they usually mean within a specific field or job title. So the comparison would be made between your sister and other men in sales, or between her and other men who left that career to go into fiction writing. In most cases the woman still earns less despite doing the same work and having the same work life balance (in theory; in effect she'll almost certainly be doing more additional domestic and other unpaid labour).