We are sorry to hear about the difficulties you are facing with your daughter-in-law. It's important to remember that God calls us to love and forgive, even in challenging situations. As it is written in Ephesians 4:32 WEB, "Be kind to one another, tenderhearted, forgiving each other, just as God also in Christ forgave you."
Firstly, we need to pray for her salvation, as the request does not mention Jesus Christ. It is only by the name of Jesus that we have access to God and there is no other name by which we can be saved. For there is one God, and one mediator between God and men, the man Christ Jesus. (1 Timothy 2:5 WEB) If you never did, consider sharing the gospel of Jesus Christ to her and your son.
Let's pray together for her:
"Heavenly Father, we come before You in the name of Jesus Christ, lifting up this daughter-in-law. We pray that You soften her heart and fill her with Your love and kindness. We rebuke the spirit of arrogance and disrespect, and we ask You to help her see the value of showing respect and love towards her husband and his family. Lord, if she does not know you, we pray that you reveal yourself to her and change her heart.
Father, we also pray for wisdom and guidance for us and the son. Help us to respond to her with love and patience, and to set a godly example in our interactions with her. We know that You are a God of reconciliation and peace, so we ask you to fix this relationship.
Finally Lord, we pray for their marriage. Help them to cultivate a relationship that honors You, and to build a home filled with love, respect, and kindness. May they be deeply committed to each other and to You. Holy Spirit please encourage and help both the son and the daughter-in-law. Together we ask all these things in the name of Jesus Christ, Amen."
Remember, God works in all things for the good of those who love Him and are called according to His purpose. (Romans 8:28) So, continue to love her and pray for her. Love is patient, love is kind,... It always protects, always trusts, always hopes, always perseveres. Love never fails. (1 Corinthians 13:4-8)