What is the religious ruling on concealing a hereditary disease (bipolar disorder) present in the family from a suitor, and is it obligatory to disclose it even though it is a tribulation specific to the brother, and is concealing it considered a betrayal of the other party?

It is not necessary to inform the suitor about the brother's illness, nor should it be done. This is because the obligation is to inform the suitor only about the defects present in the wife that would prevent the husband from enjoying her. Contagion is merely a suspicion upon which nothing is built; illnesses do not transmit on their own except by Allah's decree and destiny, as the Prophet (peace and blessings be upon him) said: "There is no 'Adwa (transference of disease without Allah's permission) nor Tiyaara (evil omen)."

The meaning of the Hadith is that nothing transmits by its nature, as people in the pre-Islamic era of ignorance used to believe. Rather, it is Allah who causes illness and grants healing. Therefore, the sister should proceed with the marriage and not hesitate.