Is it obligatory to inform a girl whom a suitor wishes to marry that she must observe the legal hijab in front of her brother through suckling or through marriage, knowing that she was raised with him as her brother? And does this situation affect the reputation of the household?

If there is no prohibited degree of kinship between the girl and the young man, whether by lineage, suckling, or marriage, then she is a stranger to him. The marriage of the girl's father to the young man's mother is not sufficient to make her a mahram to him. She must observe hijab in his presence and not be alone with him. She should be alerted to this jurisprudential error. If you marry her, you are not permitted to allow her to treat him as a mahram, and this should be clarified to her and her guardian before the marriage contract. As for whether this situation disgraces the family, it does not legally prevent marriage from them if the girl is pleasing in her religion and character.