I don't really see an issue with that. An aasimar is a person with some celestial heritage in the family tree, or that was born on an upper plane, or something like that. There's a bit of heavenly power there, but not all that much. A celestial bloodline sorcerer, on the other hand, is someone who might start out with less obvious descent from their heavenly ancestors, but they nurture that connection and feed it, making it grow far stronger than someone who relies on their birthright but focuses their attention elsewhere.