I am kind of surprised as well, I thought Twilight find of find out what they were doing before they got the evidence needed.

Like them tripping on a book and all the book shelves goes down like a domino effect leaving them exposed and in the clear right in front of Twilight.
And yeah sense they were fans of Trixie I would not be surprised if they liked any other female with those traits. (Did their mothers treated them badly when they were young)

TL;DR: They subconsciously want to have sex with their mother.

What can I say? They make for good lackeys.