We understand your concern and the difficult situation you're facing. Firstly, let's remember what the Word of God says about our relationships and finances. In Romans 12:18, WEB, it's written, "If it is possible, as much as it is up to you, be at peace with all men." However, this shouldn't come at the expense of your financial well-being or peace of mind.
Regarding your sister's behavior, Galatians 5:22-23, WEB, lists the fruit of the Spirit, which includes self-control and goodness, not self-pity and manipulation. It's important to pray for her and your relationship, but it's also crucial to set boundaries.
About the wedding, 1 Corinthians 10:31, WEB, says, "Whether therefore you eat, or drink, or whatever you do, do all to the glory of God." This includes how you spend your money. If attending the wedding would put you in financial strain, it might not be the best way to glorify God.
Let's pray: "Father, in Jesus' name, we come before You, seeking Your will for BlessedChristian. Give them the wisdom to make the right decision about attending the wedding. Help them to set boundaries in their relationship with their sister, boundaries that are filled with love and respect, but also allow them to live in peace and financial stability. We pray that their sister and niece will understand and that their relationships will be strengthened, not harmed. If this relationship is not beneficial to BlessedChristian, we pray that You will help them to let go peacefully. In Jesus Christ's name, we pray. Amen."
Remember, it's essential to seek God's will in all situations. He promises us in Romans 8:28, WEB, "We know that all things work together for good for those who love God, for those who are called according to his purpose." Trust that God has a plan and a purpose for this situation.
Lastly, we noticed that your request does not mention the name of Jesus Christ. It's important to remember that it's only by His name that we have access to God. As it's written in Acts 4:12, WEB, "There is salvation in none other, for neither is there any other name under heaven, that is given among men, by which we must be saved!" Always approach God through Jesus Christ, our Lord and Savior.