Let's bring this situation to the Lord in prayer, seeking His intervention and guidance for your sister-in-law, your brother, and your family.
Firstly, it's important to remember that God calls us to love our enemies and pray for those who persecute us, as difficult as that may be. We should also not be surprised when we face hatred, because Jesus Himself warned us that this would happen and told us how to respond. And this situation is no different.
The scriptures tell us: Bless those who persecute you; bless, and do not curse. Rejoice with those who rejoice. Weep with those who weep. Be of the same mind one toward another. Don't set your mind on high things, but associate with the humble. Don't be wise in your own conceits. Repay no one evil for evil. Respect what is honorable in the sight of all men. As much as it depends on you, be at peace with all men. Don't seek revenge yourselves, beloved, but give place to God's wrath. For it is written, Vengeance belongs to me; I will repay, says the Lord. Romans 12:14-19, WEB.
Therefore, we can pray something like this:
Heavenly Father, we come before You in Jesus' name, lifting up this situation with our sister-in-law. We pray that You would change her heart, Lord, and show her that we are not the enemy. Help her not to feel threatened or resentful towards our family, but to see us through Your eyes, Lord and love us with Your love.
We pray for healing from any mental issues she may be dealing with, Lord, and for wisdom for our brother as he lives with her. We pray for your Peace, Lord and that we all would be a light to her and others to show them the way to eternal life in Christ Jesus. Guide us, Lord, in how to respond to her, and help us to bless her and not curse her. Help us to love her as You love her, Lord, and to be at peace with her as much as it depends on us.
And lastly, Lord, we pray for our brother, that You would give him strength and wisdom in this situation. Help him to love his wife as Christ loved the church, and to be a godly example to her. Guide them both, Lord, and bring them both to Jesus Christ.
And if there is anything in our lives, Lord that is not pleasing to You, reveal it to us, convict us, and grant us the grace to repent and to obey You.
Thank You, Lord, that You are able to do exceedingly abundantly above all that we ask or think, according to the power that works in us, Ephesians 3:20, WEB.
In Jesus' name, we pray. Amen.
Now therefore ye are no more strangers and foreigners, but fellowcitizens with the saints, and of the household of God; And are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone; In whom all the building fitly framed together groweth unto an holy temple in the Lord: In whom ye also are builded together for an habitation of God through the Spirit. Ephesians 2:19-22, WEB.
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. 
