We are truly saddened to hear about the situation your brother is in. Firstly, we want to thank you for reaching out and seeking prayer for him. It is crucial to understand that the relationship your brother is in is not healthy or safe, and it is important to address this situation with urgency and wisdom.
First, we need to look at God's Word and understand clearly the truth of real Jesus Christ based courtship in marriage. For starters, nothing in Scripture condemns a woman who is submissive, gentle, quiet in spirit, loving, respectful, and supportive of her man consistent with a truly saved Christian husband. Godly marriage is ordered by God to be between a man and a woman, not the other way around. The man is the head of the wife as Christ is the head of the man (1 Corinthians 11:3 WEB). The husband representing Jesus Christ and the wife representing the Church. Husbands are to love their wives as Christ loved the Church and gave Himself up for her (Ephesians 5:25 WEB). His wife is to submit to her husband as to the Lord (Ephesians 5:22 WEB)
But in regard to Godly Courtship, God has the same requirements. When looking for a future mate, do so only as friends and with consideration and respect. It firmly believes that physical intimacy belongs only in marriage. It values honor, purity and the holiness of God. It seeks to protect as well as to please. God commands us to keep the marriage bed pure, avoid fornication. A man and a woman should be looking to marry a person with faith in God and His son the Lord Jesus Christ and not merely finding a partner. Do not be unequally yoked together with unbelievers. For what fellowship has righteousness with unrighteousness? And what communion has light with darkness (2 Corinthians 6:14)? Lastly the absolutely most important aspect of a Christian marriage is that both are truly saved followers of Jesus Christ. This means that the greatest tragedy for both is not experiences in this life but whether they spend eternity in Heaven or Hell after they die. This is the absolute most important determining factor.
You mentioned that your brother's girlfriend goes so far as to injure him with her physical violence. Which is a strong indication that she is most likely not saved. The Scriptures have a strong warning against this: A person's own folly leads to their ruin, yet their heart rages against Yahweh (Proverbs 19:3). True wisdom is just the opposite: :Whoever is slow to anger is better than the mighty; whoever rules their spirit, than one who takes a city (Proverbs 16:32 WEB). This wisdom that can come to her only if truly born again and believing in Jesus Christ who alone can save her.
If he is truly a Christian, he must believe that he must:
1 Submit yourselves therefore to God. Resist the devil, and draw near to God, and he will draw near to you. 8 Draw near to God, and he will draw near to you.
She attacks him with her hands, hitting him, clearly she is not gentle and quiet as a good wife must be:
A mild answer turns back wrath but a harsh word stirs up anger (Proverbs 15:1 WEB). Most likely out of her mouth comes a gushing fountain of impure words not speech that heals and blesses. When words are many, sin is not absent, but he who holds his tongue is wise (Proverbs 10:19 WEB).
This verse should be another red flag for your brother. No matter the circumstances, physical abuse is never acceptable, and it is not something to be tolerated or ignored. As it says
Like a madman who throws firebrands, arrows, and death, is the man who deceives his neighbor and says, I was just joking (Proverbs 26:18-19). He should be seek to remove the source of the conflict and not put himself optionally in danger.
Your brother needs to understand the severe nature of this relationship. Only God can provide us with the forgiveness, mercy, grace and help that we need to overcome great burdens in our lives. Only God can lift us up out of the depths. Continue steadfastly praying for your brother that he will listen and act, and that God will help him to see them. God can and will do a mighty work in their lives.
The most important thing that we want to emphasize is that the situation is toxic, dangerous, and should not continue. In a good marriage or courtship, there will be no physical abuse. As Christians, "love is patient, love is kind. It does not envy, it does not boast, it is not proud. It does not dishonor others, it is not self-seeking, it is not easily angered, it keeps no record of wrongs. Love does not delight in evil but rejoices with the truth. It always protects, always trusts, always hopes, always perseveres" (1 Corinthians 13:4-7 WEB). Clearly this is not consistent with her actions.
Let us pray together for the situation.
Heavenly Father, we come before You today in the name of Jesus, lifting up this brother and his girlfriend to You. Lord, we pray for a healing touch upon him, that You would mend his injuries and bring comfort to his heart. We pray that You would open his eyes to see the truth of this relationship, and that You would give him the wisdom, strength, and courage to make wise decisions based on God's Word.
We also pray for her to have a radical regeneration, in Jesus Christ, by the Holy Spirit. Please help her to see the detrimental error of her ways. Please help her man to be prayerful, to turn the other cheek, and to speak to her with love and concern. Please help him to be kind and patient to her, whether she responds in kind or not. May he man be honest with the reason of why this cannot continue, and the wrongs that have been done, combating every ill deed with a prayer on her behalf to God for her redemption.
Lord, we ask for Your intervention in this situation. Guide the words to be spoken. Guade them by your Spirit to the Church that can help on what to say. Show us how to put ourselves in their place for perspective. And use this suffering to bring Your Son glory.
We pray this all in Jesus' name, Amen.
Encourage your brother to seeking godly counsel from his pastor or a trusted Christian mentor immediately. They can help give much more personal godly counsel. It is also crucial to contact local authorities if the abuse continues, for his safety and hers.