
No, the current duchess is not the biological mother of both ML and that blond. They're the children of the late duchess, ML is the legitimate child of the previous sovereign lords while the blond is the illegitimate one as the late duchess was forced and raped by the previous emperor. They're related by maternal blood so they're half siblings.

Also, she got some disappearance from the story and comeback as the illegitimate child. She's sly and the current duchess is in favor for her due to being commoner and that two face skills she have sth like that but thanks to ML the truth uncovered and she was scorned by wearing the FML wedding dress and destroying it lmao.
She's coveting her own half sibling.