[@Rune_Alchemist]
Just an idea, but since we both use blood magic, perhaps...
Celia's father was a good man, just a little week of willed, while her aunt (mothers sister) was a selfish and jealous temptress. 
Before Celias birth, her father gave child to her aunt behind his wife's back. 
Neither wanted to own up to the deed or raise the child so he was sent away for adoption.

Hence the two characters share a father, and their mothers are sisters.
The relation a hidden and lost secret???