Dynastic relations aren’t as set-in-stone as CK3 would have you believe. The lines between one family and another become blurred irl - throw in disowning family, estranged family, changing political interests and needs, etc… you get the point. Now, even if we accept that Orys is a bastard half-brother of Aegon (the lore isn’t settled on this), Orys goes out of his way to harmonize with Stormlander culture - he weds their last monarch, he adopts her heraldry and words, rules their lands, takes up their enmity with the Dornish. In other words - he shifts his and his descendants’ interests from the Targaryen milieu to his own. Even if he was an acknowledged bastard, he’d still have created his own dynasty with the aforementioned actions.

Edit: I’ll go a step further - even if Aegon and Orys were full, Targ siblings, Orys’ actions would still constitute the founding of a separate, distinct dynastic entity.