I married my brother's widow after God spoke to several pastors and the widow herself that I must marry her. She has four children.
The problem with this marriage is that my wife has a lot of feelings and I don't have any. When we want to have sex, I must think of anything that will arouse me. My wife doesn't believe me and she is threatening to go away with the children. Is my marriage vow valid as I did not marry out of love but out of fear?
First, God said that He only speaks through His Son. "God, after He spoke long ago to the fathers in the prophets in many portions and in many ways, in these last days has spoken to us in His Son, whom He appointed heir of all things, through whom also He made the world" (Hebrews 1:1-2). These words were recorded for us in the New Testament by the apostles and prophets of the first century. "Beloved, while I was making every effort to write you about our common salvation, I felt the necessity to write to you appealing that you contend earnestly for the faith which was once for all handed down to the saints" (Jude 3). There is no on-going revelation. The message was given once for all people for all time. This is why Paul stated that the miraculous gifts, including prophecy, would end. "Love never fails. But whether there are prophecies, they will fail; whether there are tongues, they will cease; whether there is knowledge, it will vanish away. For we know in part and we prophesy in part. But when that which is perfect has come, then that which is in part will be done away" (I Corinthians 13:8-10). These preachers were not relaying a message from God. They were only giving their own opinions.
Second, regardless of your current feelings about the marriage, you entered into a covenant relationship with your brother's wife. That covenant is binding as long as you and she are alive (Romans 7:2-3). Claiming that you did not love her when you entered the covenant just means you acted foolishly, but it does not release you from your covenant. "And He answered and said to them, "Have you not read that He who made them at the beginning 'made them male and female,' and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh'? So then, they are no longer two but one flesh. Therefore what God has joined together, let not man separate"" (Matthew 19:4-6). You freely entered a covenant of marriage before God. God bound you to this woman and you cannot decide later that you want to be unbound.
Finally, love is a choice, not a feeling. You are commanded to show love for your wife because she is your companion. "Nevertheless, each individual among you also is to love his own wife even as himself, and the wife must see to it that she respects her husband" (Ephesians 5:33).
I'm sorry that you don't find your wife sexually stimulating, but you are wrong in claiming that you have to think of other things (or people) to get sexually aroused. To lust after other people (desiring to have sex with them) is just as bad as actually committing adultery. "But I say to you that everyone who looks at a woman with lust for her has already committed adultery with her in his heart" (Matthew 5:28). Stop looking for reasons not to love your wife.