We are truly sorry to hear about the distressing situation in your family. Let's bring this before the Lord in prayer and seek His guidance and intervention.
Firstly, we commend you for your perseverance in prayer. As it is written in Luke 18:1 WEB, "He also told a parable to them that they must always pray, and not give up." Do not grow weary in doing what is right and praying without ceasing.
However, we must address something crucial. You mentioned that you pray every day, but we must ensure that our prayers are directed to God the Father in the name of Jesus Christ, our Lord and Savior. As Jesus said, "Whatever you will ask in my name, that will I do, that the Father may be glorified in the Son. If you will ask anything in my name, I will do it." (John 14:13-14 WEB). It is only through Jesus Christ that we have access to the Father. There is no other name by which we can be saved. (Acts 4:12 WEB).
Now, let us address the situation with your brother. It is clear that his behavior is not aligning with biblical principles. The Bible tells us to honor our mothers (and fathers), as stated in Exodus 20:12 WEB, "Honor your father and your mother, that your days may be long in the land which Yahweh your God gives you." Your brother's actions, threatening and disrespecting your mother, are not pleasing to God.
Moreover, if your brother is not acting as he should, it is not your responsibility to fix him, but to pray for him and trust God with the situation. As it is written in Romans 12:18 WEB, "If it is possible, as much as it is up to you, be at peace with all men." Remain at peace with him as it depends on you.
Let us pray together.
Dear Heavenly Father,
We come before You in the name of Jesus Christ, lifting up this family before Your throne of grace. Lord, You know the struggles they are facing, the pain they are enduring, and the burden they are carrying.
Father, we pray for this brother. We ask that You would soften his heart, change his behavior, and bring him to a place of repentance. May he come to know You, Lord, if he does not already. We ask that You would heal his mind and emotions, and bring peace to this home.
Lord, we also pray for the mother. We ask that You would give her strength, wisdom, and courage. Help her to make the decisions that are best for her and her family, and guide her in the way she should go. If it is Your will, Father, help her to move with her child if that is what is best for them.
Father, we also pray for the one who submitted this request. Lord, give them peace, comfort, and guidance. Help them to trust in You, and to know that You are working in this situation, even if they cannot see it.
In the name of Jesus Christ, our Lord and Savior, we pray. Amen.
Lastly, consider involving wise counsel from your church or pastor. They can provide guidance and support during this challenging time. Remember, God is faithful, and He will not leave you nor forsake you. (Deuteronomy 31:6 WEB). Trust Him. Continue in prayer, and seek His will in all things. Be encouraged in 2 Chronicles 7:14 WEB, then if my people.
Prayer Focus: God, Thank You. Thank You for loving me. God, I ask You in Jesus’ name bless me with everything that I stand in need of and everything You want me to have. Bless me to know You in truth, fall in love with You with all my heart, mind, soul, body, and strength and never fall out of love with You. Bless me to have an ever growing closer stronger, more intimate relationship with You. Bless me with the love, desire, strength, and the spirit of obedience to always delight myself in You, seek first Your kingdom, and Your righteousness. Help and strengthen me God to always respect and obey You. Bless me to trust You with all my heart, acknowledge You in all my ways, and lean not to my own understanding. 
