Is it obligatory for a girl suffering from a hereditary disease to inform her suitor about her illness, and is her sister sinful for not informing her husband about her illness four months after their marriage?

If the illness affects the acceptance of the marriage or is a repulsive defect, then the suitor must be informed of it, otherwise, it is considered deception. However, if it is a minor defect that does not affect acceptance, then there is no obligation to mention it. It is permissible to inform him of it with the condition of confidentiality, and he is bound to fulfill the condition.