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Amal created a topic of Trash of the Count's Family

Can someone explain this for me so how exactly is the crown prince related to the mayor? I don’t know if I missed something but if the girl is his grand daughter is the girl then related to the crown prince as well ? Like I saw the sentence about this being the maternal side of his family. Also can someone explain how the prince is exactly 1/4 elf and why his siblings don’t use this against him since he said that his brothers and mother know about his elf side ( im guessing then that they are all also part elf tho)