I have a unique question regarding marriage. I have known a half brother and half sister to love each other very much. Their relationship is unique as the question they have, but please understand that their intentions are pure. They want to know if it is acceptable for them to be married, of course with the guide lines that they would never have any sexual conduct with each other. What they feel for each other is not at all incestuous as they say, but rather a desire to serve each other for the rest of their lives. They feel like they want that covenant of marriage as further unification with each other and the Lord. They also want to adopt a child and raise the child together as the child's parents. As I have said, there really is nothing sexual involved here. It is a godly love, but they also want to honor God and His word. They have not found any answers to their question for the Bible, and are currently fasting from food for forty days and praying over it. But they do want an elder's advice and input on their situation. Can you please help with the answers? Thank you.
That is indeed a hard question to answer. I am not an elder of the church, merely a poor scholar of the Word of God, but I will try to give some insight.
First, I want to say it is commendable that they want to "serve each other for the rest of their lives." Many siblings go separate ways and hardly speak to each other because of animosity or apathy. To see siblings who want to stay together is nice when we hear so much of family rivalries and fights. And it is certainly commendable to want to adopt and raise a child. To give a child a family is certainly something God approves.
One question that would be raised, if not beforehand then afterward, would be how the public would view such a family unit. Would a child raised in those circumstances be subjected to ridicule or teasing? That has to be considered. It would not necessarily be a deciding factor, but should be included. Of course, the same question has always been raised about interracial marriages. That doesn't mean that two people of different races shouldn't marry, just that they have to consider the children.
The main consideration, though, should be what the Bible says about marriage. Particularly, what is the purpose of marriage in the Bible?
One reason siblings, including half siblings, were forbidden to marry under the Law of Moses was just the issue you brought up, the issue of sex. There was no law against siblings living together as siblings for their entire lives if they want. They were simply forbidden to marry. (Most civil laws forbid marriage of two individuals that closely related by blood, as well.)
The basic reason for marriage throughout the Bible has been to provide a family unit for two basic purposes: so that a couple could have a safe, stable sex life; and so that any children that result would have a family to raise them.
"Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh." (Genesis 2:24) The idea of "one flesh" is often considered to include sex.
"The same day came to him the Sadducees, which say that there is no resurrection, and asked him, Saying, Master, Moses said, If a man die, having no children, his brother shall marry his wife, and raise up seed unto his brother. Now there were with us seven brethren: and the first, when he had married a wife, deceased, and, having no issue, left his wife unto his brother: Likewise the second also, and the third, unto the seventh. And last of all the woman died also. Therefore in the resurrection whose wife shall she be of the seven? for they all had her. Jesus answered and said unto them, Ye do err, not knowing the scriptures, nor the power of God. For in the resurrection they neither marry, nor are given in marriage, but are as the angels of God in heaven." (Matthew 22:23-30) Clearly the principal reason for marriage was procreation, having children through sexual union.
"Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge." (Hebrews 13:4) This shows marriage is the only place for sex, and that that is one of the purposes for marriage.
The entire chapter of 1 Corinthians 7 is about sex and marriage. In particular, Paul says that a principal reason for marriage is so that a couple not burn with lust for each other, but get married so that lust can be properly fulfilled. Paul specifically tells a married couple not to "defraud" one another by refusing sex. In the situation you describe, if the siblings were married (which is probably illegal to start with), and one later decided that the marriage allowed sex between them, then the other would be sinning by denying the incestuous desires of the other. But giving in, to avoid sinning, would also be sin. It could put one in a no-win situation, which certainly would not be the will of God. You may say they don't have those desires, but close proximity in a marriage can change one's original intentions.
What constitutes a marriage? In the eyes of many religions and some laws, a marriage is not a marriage until the couple engage in sex. In the Catholic church, for instance, as well as in the laws of many states of the United States, a marriage may be annulled (considered as if it never existed) if the parties can show it was never "consummated" by sexual union.
A "marriage" would add nothing to their covenant to live together and raise an adopted child. Because of the biblical purpose for marriage it could actually cause more harm. It could also cause misunderstandings with people who knew they were married and then found out they were siblings.
I hope these ideas help.