Unlike most Fullers, Sera didn't have the Fuller's metal related bloodline but a strange one, which she had probably inherited from her mother, who was the non Fuller parent.
Though Riva was surprised to find this because unless her mother had a higher tier bloodline than Fuller bloodline–at least one on the same tier as the Royal Family's bloodline–the offspring would always inherit the Fuller Bloodline, she wasn't in mood to think about Sera's dead mother's origin because she realized Sera, who she had come to think of as her daughter, was slowly dying.
The reason: Her strange bloodline was slowly eroding her lifeforce.
She used many methods but she couldn't stop her bloodline from eroding her lifeforce.
Not wanting her to worry about it, she decided against telling her. And even though she felt guilty, she dropped the idea of taking her as her disciple and stopped guiding her cultivation.
The 10 Bonus Chapter Mass release between 10-15 November!