There are many misconceptions regarding Andraste's bloodline, monsieur. This is due, I should think, to a general lack of knowledge regarding Andraste's mortal life. Understandable, considering the many cults that arose following her death. Every one sought to claim Our Lady for their own culture or claimed some aspect of her existence was a lie—all of them complete fabrications. My order has done considerable research to ascertain the truth.
We all know Andraste and the Betrayer raised five children. The eldest three were sons: Isorath, Evrion, and Verald. The rule of what was once southern Tevinter was split among them. Isorath was given the west, what is today Orlais. Evrion was given the east, what is today the Free Marches. Verald was given the central Planasene, what is today Nevarra. What became of these men and their legacies is the stuff of legend, and the majority of claimants to Andraste's bloodline link back to one of them. None of the three sons, however, were born of Andraste. They were born of the Betrayer's concubine, Gilivhan. People choose to overlook the fact that Andraste came from the Alamarri tribesmen and that they were barbarians, not the Fereldans we know today. They were savage warriors who took concubines in addition to their wives, and because Andraste was thought to be too weak to bear children, the Betrayer took Gilivhan to provide him heirs. Which she did. After her death, the sons were raised as Andraste's own.
Later in life, Andraste proved predictions wrong and had two daughters by the Betrayer: Ebris and Vivial. They were kept out of the public eye and not permitted to marry, though both had consorts. Ebris had but one child, Alli Vemar, who perished on a voyage to Denerim—less than a month after her mother fell to plague, and without children of her own. The younger daughter, Vivial, was more controversial: a strong-headed woman who defied her family by falling in love with a mage of Tevinter, Regulan. Vivial and Regulan went into self-imposed exile as the Exalted March began, and into hiding following Andraste's betrayal and murder.
What became of Vivial and her descendants is largely unknown for one primary reason: she had only daughters. Each of those daughters only had daughters. They married into other families and took other names, and in the chaos of the Second Blight, all traces of survivors were lost. Andraste's true bloodline, if it exists, lies solely in the descendants of Vivial, and the suspicion of my order is that it produces only daughters. Thus the claims of your young man, monsieurs, are highly suspect.
—From a letter by Sister Galenna of the Augustan Order, Dragon 9:12