So what? Harry never married the Junkie he impregnated with Aisling either... Considering he's disinherited on top of that, I'm still surprised she's even mentioned.
Well, the Empire's obviously building up to a succession war, and a fairly nasty one too considering how many factions there are. You only need a pretty vague claim to the throne to be a pretender if you also happen to have an army to back that claim up with; even if it's blatant rubbish and everybody knows it, it'll be accepted as legitimate if you win the civil war because, well, you've won the civil war and now you're the only one with an army.
Aisling happens to have an army, therefore she's a 'legitimate' pretender, no matter how spurious her claim might or might not be in the technical legal sense.