Stardust (novel)/WMG

Una was a Witch
Either she was one before she was sold into slavery, or she picked up some tricks from Ditchwater Sal. Chances are, she had some magical ability -- she's the film's only reliable source of Babylon Candles, which would logically require magical talent to create.
 * I thought all witches in Stardust are Always Chaotic Evil perhaps she just stole a few from Ditchwater Sal's personal stash.
 * A. If they were Always Chaotic Evil, Ditchwater Sal wouldn't have honored her bargain with Tristan; she may have turned him into a mouse, but she did give him food, lodging, and safe passage to Wall, setting him free when he got there. B. Babylon Candles are a valued commodity, and apparently rare enough that if you had one, you'd keep track of it; even the three queen witches can't scrounge one up on their own, it seems.
 * Though I agree with the above, that Sal was hardly outright evil (or, at least, not the same league evil of the Queen witches) I would like to point out: she did give her word, which might mean a lot more in magical kingdom like the one from Stardust.
 * Indeed. The book in particular frequently mentions magical rules governing exchanges and promises. However, Sal does still seem more of a Neutral bent than Evil anyway. Didn’t particularly profit her to betray Tristan, so no reason not to keep the bargain.
 * Sal seems just as willing to murder a star to restore her youth as the Lilim was, though she never got the chance to even try, so she wasn't exactly nice person (let alone about that bit about enslaving a child), though just being a magic user doesn't seem to automatically mean "evil" in the book.
 * Isn't this one actually canon? According to the book, magical ability had run through the family blood for some generations now. It's downright stated that Tristan's knowledge of the fantasy landmark's locations was of a magical origin.
 * It definitely runs in Una's family- in one of the chapters that centers on Septimus he laments that he doesn't have more magical power, but only some of the "locating magic" that runs in his family. It's one of the hints as to Tristran's identity.

Una had other reasons for seducing Dunstan.
With seven older brothers trying to do each other in, she just deemed it unlikely that she would ever get the throne. She probably felt a little neglected, too, because males were prioritized in Stormhold politics - meaning, she could never get the throne from her brothers, but as a female she did have the power to bear a son that could usurp them. Once she was captured by Sal and met Dunstan, she thought him a good sire for her future son. And it worked.
 * One problem. She was kidnapped as an infant.
 * In the movie, she wasn't; she recognizes Septimus.