The Westermarck effect attempts to explain why that is not a simple mental adjustment. It's a theory, but it states "people who live in close domestic proximity during the first few years of their lives become desensitized to sexual attraction." [1]

[1] https://en.wikipedia.org/wiki/Westermarck_effect

Maybe this is easier if you've had a sister. I never did. And I can't sit across the table from a skin-flashing admin assistant and think of her like my mom, who is in her 70s. It doesn't work.

The fact that you use the term "skin-flashing" here is a little concerning. Perhaps it was just meant as a shorthand for attractive woman, because otherwise you've taken a woman's choice of dress and behaviour and made them part of your neurosis. Unless of course she is giving you the Basic Instinct treatment (is that too old a reference for this site?), in which case I'm entirely out of advice for you.