'Soreia Eyeris' was actually born under a different name, to no one of importance—just another orphan taken in by a mysterious organization.
No. She wasn't adopted—not legally, anyway. The real, biological Soreia Eyeris was long dead. She'd died around the age of five—though everyone except for her 'parents' in the Eyeris family remained unaware of that fact.
The Eyeris family was unique as far as beast-tamer families went. In most noble families, a person's status among the next generation was determined by the quality and potential of their affinity.
Cassian, for instance, was considered a cut above his competition for the throne due to his broad and powerful affinity with multiple high-ranking dragon species. It was why he had been prematurely named crown prince before even advancing to the level of a 5-star beast-tamer.