I haven't actually tried this, not even tried to kill her without Nalia, (unless that one time when Valen broke out of the formation, she is a feisty one, isn't she?) but surely Nalia must react if you attack her with her in the party? Maybe if Valen threats her, and Nalia ask you to choose side. Kill auntie and Nalia, or a vampire who decides to feast.
She could at least be upset, or maybe she doesn't like her aunt? She obeys "etiquette" like the rest of them, like her father, but maybe Nalia in secret is not as fond of her aunt like she pretends to be?
Perhaps Nalia, the mage is behind all this, an agent for the Twisted Rune, who makes the Roenalls look responsible for numerous deeds (taking her keep, killing her father, holding her hostage), while Nalia in turn (in company of the PC, of course) kills the pathetic slavers who are deemed soooo unworthy as a puppet of the Rune, and eradicating the Roenalls, a strong threat against her land. Maybe Nalia herself sent Torgal to the keep in the first place. (although this assumes you're a fighter who inherit the keep).
Alright, I'll stop my speculations, or I'll surely end up in Spellhold with the other lunatics.
I'm NOT interested in the opportunity to listen to Tiax every now and then (Irenicus did him a favor).