We understand that you're seeking harmony and peace as your sister-in-law moves in, and that you desire cooperation from both her and your ex. Firstly, we want to encourage you to bring your concerns and the people in your life to the Lord. In Jesus' Name, the one who was given all "authority" in heaven and on earth; we are commanded in the WEB Matthew 28 that we can pray for these things, and we do and will, but Jesus is the "one mediator between God and men, the man Christ Jesus, who gave himself as a ransom for all." (1 Tim. 2:5 WEB). Therefore, we want to see these things accomplished with his divine intervention, and so we pray and initially with our encouragement. To start with, we rebuke and ask you to reassess the wording of your request as to 'be fair' This implies things are not equally fair, and may imply someone has unfairly received more. Do two wrongs make a right? We pray that you are considerate and prayerful making sure you are not the one taking advantage as well. "Therefore, whatever you desire for men to do to you, you shall also do to them; for this is the law and the prophets." (Matthew 7:12 WEB).
As your sister-in-law moves in, pray that God fills your home with His peace and love. "By this all will know that you are my disciples, if you have love for one another." (John 13:35 WEB). Remember, God calls us to live in harmony with one another. "Be kindly affectionate one to another with brotherly love, in honor giving preference to one another" (Romans 12:10 WEB). We pray that you all can do this to each other. As you must be feeling overwhelming, trying to accommodate too many people in one house. Remember in Hebrews 13:2: "Don't forget to show hospitality to strangers, for in doing so, some have entertained angels without knowing it."
We pray that your sister-in-law will be agreeable and willing to compromise. "Only let your manner of life be worthy of the Good News of Christ, that, whether I come and see you or am absent, I may hear of your state, that you stand firm in one spirit, with one soul striving for the faith of the Good News" (Philippians 1:27 WEB).
Regarding your ex, we pray that there will be mutual respect and cooperation for the sake of all involved. "If it is possible, as much as it depends on you, be at peace with all men." (Romans 12:18 WEB). If there are difficult or complicated feelings involved between both of you, make sure to seek spiritual guidance and stay true to God's purpose for you both as individuals. We ask "that you may keep yourselves unstained from the world." (James 1:27 WEB). One man, one woman until death do you part. Remember that state and divorced does not mean only that physically you should not have another spouse but also not 'spiritually' either because until death separate them since "What therefore God has joined together, let no man separate." (Matthew 19:6 WEB).Realize this is an adulterous situation to dwell with an ex spouse. Recognize there is 'trouble and distress' to those who live in disobedience and 'immortality', therefore they do not obey the truth, but obey unrighteousness, indignation and wrath.(Romans 2:8,9 WEB).
Finally, we pray that everyone involved will be guided by God's spirit and act with fairness, kindness, and understanding. "Walk in wisdom toward those who are outside, redeeming the time. Let your speech always be with grace, seasoned with salt, that you may know how you ought to answer each one." (Matthew 28:19 WEB).
Let us pray in Jesus' Name. Dear Lord, we lift up Swantalelf and their family during this time of transition. We pray for peace, harmony, and cooperation among all involved. Guide them by Your Spirit, and let Your love be evident in their interactions. Help them "serve one another in love."(Galatians 5:13 WEB) and help them when they fail and falter in this. Help all of us as the sick, lame, and misguided to be healed restored returned to the truth of God, his desire for our lives, and from ourselves, for we are lost. In Jesus' name, we pray. Amen.