We are truly concerned about the situation your brother is in. Firstly, we want to address the issue of violence. The Bible tells us that "husbands, love your wives, and don’t be bitter against them" (Colossians 3:19 WEB), and that love is patient and kind, not arrogant or rude (1 Corinthians 13:4-5 WEB). While this is speaking about husbands and wives, and not boyfriends and girlfriends, it is clear that physical abuse, controlling behavior, and isolation from family are not in line with biblical principles. All people should be treated with kindness and respect, especially in a relationship that you consider serious.
Your brother's situation is not healthy, and it is important for him to seek help and safety. We strongly encourage him to contact local authorities or a trusted counselor to deal with the abuse. Being a Christian means you do not allow someone to control you or cause you harm, but you must be bold and stand up for the truth. 2 Timothy 1:7 For God hasn’t given us a spirit of fear, but of power, love, and self-control.
Regarding his girlfriend's past marriages, the Bible tells us that "the Lord is the avenger in all these things, even as we also told you before and testified" (1 Thessalonians 4:6 WEB). While we should not judge others, we should be wise in our choices. Marriage is a sacred institution and is a lifelong commitment between a man and a woman. In the case of your brother, we should remember the wisdom of 2 Corinthians 6:14, "Don’t be unequally yoked with unbelievers, for what fellowship have righteousness and iniquity? Or what communion has light with darkness?". If she is not a believer in Jesus Christ, then this relationship is forbidden and is a sin against Jesus. This is regardless of the behavior of the unbeliever. If she is a believer, then she is not acting like one and is in rebellion against the teachings of the Bible.
We also note that your brother's request does not mention the name of Jesus Christ. It is only through Jesus that we have access to God the Father. Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me. (John 14:6 WEB). For there is one God, and one mediator between God and men, the man Christ Jesus. (1 Timothy 2:5 WEB).
Let us pray together:
Dear Heavenly Father,
We come before You today, lifting up our brother and his situation. Lord, we pray for Your protection and guidance in his life. Help him to find the strength and courage to remove himself from this abusive situation. Be the God of the Psalms, a present help in trouble (Psalm 46:1). Father, we ask that You open his eyes to Your truth and guide him in the path of righteousness, away from people that do not follow you. If he is in a relationship against you, convict him of this and he will repent and be saved. We pray for his girlfriend, that she would come to know You and be convicted of her sin. Help her to find forgiveness and salvation in Jesus Christ. We pray this in Jesus' name, Amen.
You have not specifically mentioned if your brother is a believer in Jesus Christ. If he is not, then we encourage him to believe in the Lord Jesus Christ. For God so loved the world, that he gave his one and only Son, that whoever believes in him should not perish, but have eternal life. (John 3:16 WEB). URGE HIM TO BELIEVE IN JESUS AND REPENT OF HIS SINS. THIS IS THE ONLY WAY TO BE FREE AND TO OBTAIN HELP FROM GOD.