I want to know if it is a sin to have sexual relations with the first cousin of former spouse?
I do not know details about the case which you mean and I can not understand to the end from this question, but any sexual relationship outside marriage is a sin and a misdeed that brings serious consequences on this earth and for eternity. God says very clearly and categorically:
Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for fornicators and adulterers God will judge.(Hebrews 13:4)(NASB)
If you have divorced with your husband because of the sexual relationship with his first cousin, or with any other person – you have committed adultery and you must repent.
If your spouse died, or left you and then you have entered a sexual relationship with his first cousin, or with any other man, before you got married and had the religious ceremony – you have committed adultery and you must repent of your sin that will bring you consequences.
If now you are not part of a covenant marriage after your husband died or left you and you were proposed marriage by his first cousin, there is no commandment of God that will prohibit your marriage, if he is also a Christian.
I suppose you have asked the question because you have fear that this is a relationship of incest. In the book of the Old Testament, Leviticus, Chapter 18 are presented which are all relations of incest prohibited by God.
I suggest you to study the text carefully and come back with questions that might arise.
Translated by Felicia Rotaru