Let's bring your situation to the Lord in prayer and seek His guidance according to His word.
Firstly, we are reminded in Ephesians 4:32 to "be kind to one another, tenderhearted, forgiving one another, even as God in Christ forgave you." This verse underscores the importance of forgiveness, as we have been forgiven much more than anyone might ever hold against us. So, forgiving your cousins is not only an act of obedience to God's word but also an acknowledgment of the forgiveness you've received from God.
However, forgiveness does not mean you must accept their offer or continue in a close relationship with them if that will cause more strife or issues. Romans 12:18 says, "If it is possible, as much as it is up to you, be at peace with all men." Note that God knows sometimes it is not possible, but we should do what we can to live at peace.
Regarding your fiance, it's important to consider their feelings, but ultimately, you should be led by the Holy Spirit. God may be using them to protect you, but He also may be working in your heart to extend forgiveness and reconciliation when possible.
If your disagreements with your cousins involve matters that go against biblical principles, then it is wise to set boundaries. However, if the issues are merely personal differences or past hurts, then consider the power of forgiveness and reconciliation. It is also important for you and your fiance to agree on such matters as much as possible. Amos 3:3 says, "How can two walk together unless they are in agreement?"
We must also remember Colossians 3:13-14, "Forbearing one another, and forgiving one another, if any man has a complaint against any; even as Christ forgave you, so also do you. Above all these things, walk in love, which is the bond of perfectness."
Let's pray together:
Dear Heavenly Father, we come before You in the name of Jesus Christ, lifting up this sister who is in need of Your guidance. Lord, You know the intricacies of her situation, and You understand the complexities of her relationships. Help her to have a heart of forgiveness towards her cousins, just as You have forgiven her sins.
Give her the wisdom to know how to navigate this situation. If accepting the bridal shower offer is not in her best interest, then give her the grace to decline respectfully. If it is Your will for her to accept, then soften her heart and that of her fiance towards her cousins.
Lord, we also pray for unity between her and her fiance. Help them to be of one mind and spirit, and to make decisions that honor You. Guide them as they prepare for their marriage, that it may be a blessed union that reflects Your love and grace.
Lord, we ask all this in the name of Jesus Christ.
We encourage you to continue to seek God's will in this matter and our Lord Jesus Christ.
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. 
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