Is a Christian girl's conversion to Islam considered valid if she embraces Islam secretly for fear of her family, or must she declare her conversion despite her fear of her family's hostility, especially since she is new to Islam?

The sister is not obligated to declare her Islam if she fears harm from her family or others. It is sufficient for her to embrace Islam secretly, as Allah Almighty says: "And a believing man from the family of Pharaoh, who concealed his faith, said," and as the Messenger of Allah, peace and blessings be upon him, said to Abu Dharr: "O Abu Dharr, conceal this matter."