What should I do about my brother who watches indecent movies and talks to girls online, knowing that he prays, and how can I save him from these evil deeds?

Explain to your brother the religious ruling on looking at forbidden things and its dangers, and pray to Allah for his guidance. Encourage him to supplicate with "O Allah, Turner of hearts, keep my heart firm upon Your religion." Approach him gently to tell him that his actions are not consistent with prayer. Urge him to marry, and do not tire of inviting him to pray in the mosque and attend lessons of knowledge. Alert him to the importance of good companionship. Occupy his time with truth, for if the self is not occupied with truth, it will occupy its owner with falsehood, and idleness is one of man's enemies.