okay I think it needs to be explained because it wasn't done well enough here.
Our blonde princess is a legitimate princess.
She was born under the current emperor/king along with her brother, the current crown prince.
Her father, the current emperor/king, is the brother of the late and former emperor/king. The late and former emperor/king fell in love with the previous sainteness and redhead princess's mom.
They pa pa pa'd even though it is against sainteness religious beliefs. Sainteness becomes pregnant, is informed that the child in her stomach will die if she stayed in the palace so she runs away.
After successfully giving birth to her daughter and the original female lead, the sainteness dies.
Her daughter therefore lives as an orphan with no clue about her bloodline and birthright.
Since she came from the previous emperor/king her right to the throne should be higher than our main character and her brother. However, it is at this point that I feel there are some plotholes because I do not know why they question our main character's bloodline when she has also been proven to have the blood of the first empress.