I entered into a long-distance relationship with a man who happens to be the brother of someone with whom I had sexual relationships with more than 15 years ago. I was never married to the man, although we lived together for a couple of months. We lived in adultery and fornication because the man was still married - in the process of getting a divorce, but the divorce never happened while we were together.
My boyfriend and I truly love each other and as I said, it is long distance so we have truly been able to experience love beyond a physical attraction by getting to know each other's values and stands in different aspects of life and religion and more recently, our love is anchored in our faith. His brother claims I was his wife because he knew me intimately and therefore, deems our relationship unlawful and unclean. I truly believe our relationship and eventually marrying my boyfriend is not a sin because I was never his brother’s wife, but my boyfriend and I are very confused what the definition of a wife is within the context of Leviticus 20:21 and Mark 6:18 to name a few passages that talk about this.
I am also confused about repentance. Does my repentance from physical knowledge of this man give way to a relationship with his brother if, in fact, I was his wife within the biblical context? I mean, I cannot fix the past; I can only live from now on following God's will.
I would appreciate your feedback.
I can see why you're confused. There are numerous inaccuracies being touted which makes things confusing.
First off, you were not married to the man with whom you committed adultery. If you had been, then it would not have been adultery. A sexual relation doesn't create a marriage. It is the covenant -- the marriage vow -- which creates a marriage. "Yet she is your companion and your wife by covenant" (Malachi 2:14). The marriage and divorce situation is difficult enough to untangle these days. I would shudder to think of the mess that would have to be dealt with if each time a person slept with another created a marriage.
Second, you are not making a distinction between the New Testament and the Old Testament law. The Old Testament is no longer in force. "Having wiped out the handwriting of requirements that was against us, which was contrary to us. And He has taken it out of the way, having nailed it to the cross" (Colossians 2:14). Read "Why We Don't Follow the Old Testament" for more details. Though we are not under the Old Law, it still has some use for us (Romans 15:4; I Corinthians 10:11). When God condemned something as wrong under the Old Law, we ought to understand why it was wrong because such principles often give us insight into the New Law.
The law you cited does not apply since you and this man were not married. "If a man takes his brother's wife, it is an unclean thing. He has uncovered his brother's nakedness. They shall be childless" (Leviticus 20:21). Uncleanness does not mean sin. Breaking the laws of uncleanness was sinful, but uncleanness itself were things that most people would find disgusting. For example, pigs were unclean because of what pigs are willing to eat. See "Uncleanness" for more details about what uncleanness is. Under the Old Law, if a man died childless, his nearest brother was to marry his wife (Deuteronomy 25:5-6). Any child resulting from that union would be counted as his brother's children and would inherit from that brother's estate. Thus the law in Leviticus is not covering cases when a brother has died. Nor could it be talking about a man committing adultery with his brother's wife since that was punishable by the death of both the man and the woman (Deuteronomy 22:22). Therefore, the only way this could happen is if a man had divorced his wife, then his brother was not allowed to marry her. Such makes sense since it would create huge problems within the extended family.
It is this law that Herod had violated by marrying his divorced brother's wife. In fact, John's statement, "For John had said to Herod, 'It is not lawful for you to have your brother's wife'" (Mark 6:18), hints that in God's eyes Herodias was still married to Herod's brother since he calls her his brother's wife -- not former wife. Again, this statement does not apply to your situation.
There is another, related law, that comes closer to your situation. "Nor shall you take a woman as a rival to her sister, to uncover her nakedness while the other is alive" (Leviticus 18:18). "To uncover her nakedness" is a way of saying having sex. And the warning is not to be having sex with two sisters because it will create a rivalry between them.
This is basically what you are seeing. There was a reason why you left the man you committed adultery with. That left him with strong feelings about you. But people typically think that if they don't like someone, then everyone else around them ought to have the same opinion. He doesn't want to see his brother marry the woman who left him, either out of jealousy or hatred. The fact that he still holds these feelings after 15 years should tell you something about the strength of his feelings.
I'm not saying you can't marry the man you love; after all, we aren't under the Old Law. However, you both need to be fully aware that this man's brother is going to make things awkward and uncomfortable for you. He has already demonstrated his lack of respect for marriage vows and you didn't mention that he has changed his behavior in any way. He is likely to think he is in competition with his brother. Family gatherings are going to be uncomfortable as a result. But if you know this in advance and take steps to minimize the problems, you will probably get by. I definitely would not suggest living close to this brother after you get married.