I Made A Contract With A Dragon To Get Revenge On My Mother

Thelma and Louise were born as twin daughters of the famous Preteria family.
Among them, Thelma lived her whole life pretending to be a man, as her mother instructed. Even the sadness of her twin sister, Louise, who is neglected by her, is ultimately for our benefit. He also endured the position of marquis, which was not appropriate for his age... “Why don’t you just kill us when we were born and replace us with another boy?”
We learn that her mother, Gina, is planning to get rid of Thelma and establish her real man, the Marquis, through her sister's arranged marriage.
Just in time, a dragon, a being from the past, appeared before the twins who were cornered as a salvation. “Make a contract with me.”
You cannot make a contract with a dragon that was recorded as an enemy of the empire. but.... “Preteria. If you make a contract with me, I will kill your mother.” What is the intention of this dragon who makes an offer that is difficult to refuse?