We are truly saddened to hear about the situation your brother is in, and we are here to intercede and provide biblical guidance. Your brother is in a situation that God clearly addresses in His word. First, let's acknowledge that God hates divorce and it is only permitted in cases of adultery. We need to pray for your brother and his wife, for God to bring them back together and restore their marriage.
Your brother is currently in adultery and this is sin. God's word says in Exodus 20:14 WEB, You shall not commit adultery. Living with or being with this other woman is a sinful situation that God is not pleased with. Furthermore, the fact that this woman is related to your half-brother's wife makes this even more problematic, as it has the appearance of incestuous adultery, which is expressly condemned in God's word in Leviticus 18:20 WEB, You shall not have sexual relations with your kinswoman. ...
Let's pray for your brother and his wife.
Almighty God, we come before You in the name of Jesus Christ, pleading for our brother who has been trapped in sin. Lord, we know that You hate divorce, and we ask that You would bring him back to his wife and restore their marriage. Give him the strength to turn away from this adulterous relationship and return to his family. Lord, we also pray for this other woman, that You would convict her heart and bring her to repentance. Lord, we know that You can do all things and that nothing is impossible for You. We ask this in Jesus, name with thanksgiving and faith. Also, Lord, we pray if there is an issue of salvation for any of the parties involved, and we ask Lord that You bring them to a saving faith in Jesus Christ.
We encourage you to stand firm in your faith and continue to pray for your brother. Remind him of God's promises and the consequences of his actions. Also it is wise to get other family members involved to encourage making Godly decisions and to eliminate this sinful behavior. Let him know that God is faithful and just, and that He will forgive him and cleanse him from all unrighteousness if he confesses his sins and repents in the name of Jesus Christ. 1 John 1:9 WEB, If we confess our sins, he is faithful and righteous to forgive us the sins, and to cleanse us from all unrighteousness.
Lastly, we would like to address that we only have access to God through the name of Jesus Christ, and that it is only in His name that we can be saved. As it says in Acts 4:10-12 WEB, ... let it be known to you all, and to all the people of Israel, that in the name of Jesus Christ of Nazareth, whom you crucified, whom God raised from the dead, ... There is salvation in none other, for neither is there any other name under heaven, that is given among men, by which we must be saved!.