How can one deal with a brother who denies the existence of the Creator and questions the ثابتات of religion, influenced by a bad friend, while his younger brother is following him in this line of thought?

Denying the existence of God, distorting the Quran, and denying the obligation of prayer are acts of disbelief (kufr). This should be addressed by supplicating for their guidance, then by enlightening them with rational and textual proofs for the existence of the Creator and the Quranic verses that indicate Him. Then, by explaining the dire consequences for anyone who dies upon a religion other than Islam. They must be dealt with wisely and rationally, and an attempt should be made to separate the brother from his cousin. The issue of canine impurity is a matter of scholarly disagreement and should not be a priority in the discussion compared to matters of disbelief.