What is the ruling on the relationship between a girl and her brother whom she accuses of having looked at her [lustfully] when she was young? Is he considered a stranger to her, and is she sinful for severing ties of kinship if she avoids speaking to him and wears the niqab in front of him?

The sister's behavior and her wearing the niqab in front of her brother is appropriate, and there is no severing of kinship ties in it, especially since the aforementioned brother spies on her and looks at her with lust, is negligent in prayer, and speaks with non-mahram girls. The relationship between mahrams is not permissible if there is a fear of lust.