Either she becomes a Dame (which, still possible), or her husband becomes Knight of Spottswood by jure uxoris.
(This is the asker, by the way.)
I considered both options, and found them… wanting. I dunno, but I feel as if dames were a thing in Westeros, we’d’ve heard about them by now, and… while in any other kingdom of Westeros, I’d buy the jure uxoris answer… in Dorne, I feel like ruling ladies would be especially leery of that kind of thing, for the same reason ruling queens had prince consorts and not kings.
I don’t see what the problem is, per se.
Why haven’t we heard about them? We hear very little about landed knights period. There’s only seven mentioned in the entire series, and very few of them are significant characters who get much of a mention. But logically speaking, given that we know of some pretty-longed lived houses of landed knights, unless they never had an only-female-heir situation in their entire existance, it must have happened sometime. Also, GRRM in his SoSpartinMartins on the subject has basically said that the only difference between a lordship and a landed knighthood is the title and “powers of rulership,” not property rights. So if there are ruling ladies, I would argue the weight of evidence is toward there being ruling dames.
Jure Uxoris? If as you say, women cannot become knights (which, sidenote, are we sure is actually part of the canons of knighthood and not just a cultural expectation?), jure uxoris is better than being completely disinherited, because your kids will get to inherit.