Here is my theory:
Since every emperor dreamed of how they can steal glory of Kalisto family if they create a bond via marriage, a daughter of someone from Kalisto family was sent to another noble family to be a adopted child -so she wouldn't be target of emperor- That child is our FL's mother who is also the count is obsessed with. They mentioned about her family name but I forget so if you want to check please go back to a few chapters before. Because that family is no longer exist it also means there are no one who knows about adoption of the daughter, and the necklace is probably a symbol that show she is descendant of Kalisto family.