What is the ruling on one who does not admit his action before his father for fear of his brother cursing the religion?

Cursing religion is disbelief (kufr), and it is obligatory to advise and invite your brother [to righteousness]. He may be ostracized if he persists. It is not permissible to lie to avoid cursing religion, for lying is forbidden, and the sin of cursing falls upon its perpetrator. Abandoning an obligation under the pretext of avoiding fitna (strife/temptation) is itself fitna. The obligation is to abandon lying for the sake of Allah, not for people or to seek reputation. Righteous deeds must be exclusively for Allah alone, for whoever knows Allah dedicates their deeds sincerely to Him. What you have done by being truthful is correct, and you must be truthful with everyone.