We understand your concerns and the delicate situation you're in. Let's bring this matter to the Lord in prayer and seek His guidance and protection according to His will.
Let us come to you, for you are our God. We have no other name for salvation but Jesus. For there is no other name under heaven given among men whereby we must be saved.
First, let's address the spiritual aspect. We are not told if your sister or her family believe in Jesus Christ as the only way to salvation. If not, then that is the greatest concern and need for them. Without this, they are not at peace with God and do not have the Holy Spirit to guide them in proper living. Let us pray that they will all believe the Gospel of Jesus Christ.
Father, we come before You in Jesus' name, seeking Your protection and guidance for our fellow believer and their family. We pray that You will put a hedge of protection around them and their loved ones, guarding them from any sickness that may come their way.
We ask that You will convict their sister and her family of their need for Your guidance through faith in Jesus Christ. We ask that You will bring them to faith in Jesus Christ and to the knowledge of Your will for their lives and to act accordingly. If they are not yet your children, make them your children and therefore at peace with you.
We also pray for wisdom. Your Word says, "If any of you lacks wisdom, let him ask of God, who gives to all generously and without reproach, and it will be given to him". We need wisdom in dealing with this situation, Lord.
We have a duty to be at peace with all men and to treat them with proper respect, but we also have a duty to love our neighbors as ourselves. These two responsibilites may be in conflict with each other and we must balance them. We should not cause others to stumble and fear for their health and their lives. Let us approach others with kindness and respect, but with firmness and resolve. We must speak with love, but also with truth.
We also ask that You will help our fellow believer manage their time and responsibilities wisely, especially concerning their schoolwork and the potential visit from their niece.
Regarding the concerns about exposure to Covid-19, Your Word reminds us, "For God has not given us a spirit of fear, but of power and of love and of a sound mind". We pray for wisdom and discernment to act with prudence and caution, but not with fear.
We ask that You will work in their sister's heart and help her to understand the seriousness of the situation and the importance of being careful. If they are a believer, then help them to consider the words of the apostle Paul, Or do you not know that your body is the temple of the Holy Spirit who is in you, whom you have from God, and you are not your own? For you were bought at a price; therefore glorify God in your body and in your spirit, which are God’s.
Therefore, rebuking with all authority, let him who thinks he stands take heed lest he fall.
Let us encourage one another to be steadfast if we are to suffer or to be in distress. We are more than conquerors.