It is quite strange that Aenys Targaryen would be paired with Alyssa Velaryon as a political match. The two houses were already close allies, Aenys' grandmother herself was Valaena Velaryon. That brings up the question as to why Alyssa was a "political match" for Aenys. In my belief, there is only one possible reason why the two would be coupled for political reasons. Either Alyssa or her mother (a member of House Targaryen) had hatched a dragon. I believe that this other junior branch of House Targaryen was dragonless, as at the time of the Conquest only Visenya, Aegon and Rhaenys had dragons. I think it is more likely that Alyssa's mother would have hatched a dragon (perhaps Vermithor) being the first of her junior line to do so and after her marriage into House Velaryon. Aegon and his sisters would have been eager to not have House Velaryon as a second dragonriding house and potential rival. I think it is then likely Alyssa was the only one of her siblings to hatch a dragon herself (perhaps Meleys). Alyssa was then betrothed into House Targaryen's main line to prevent the spreading of the "dragonrider gene".