How Do I Save My Marriage From Imminent Collapse?

How Do I Save My Marriage From Imminent Collapse?
Dear Agatha,
Please help me before my marriage suffers irreparable damage. Early last month, I came back from work to find my wife’s younger sister in the living room naked making love to herself with a vibrator and an adult film playing on the television.
The scene was too much for me. I didn’t know when I joined her on the floor and made love to her. I only realized my mistake after the whole thing was done.
I tried apologizing to her but the words got stuck in my throat due to embarrassment. She on the other hand was enjoying the whole episode and took advantage of my confusion to plant a big kiss on my lips. Fortunately my sanity was restored and told her to stop it.
She didn’t try to persuade me. She instead took the vibrator and continued what she was doing before I came in.
I immediately took off to my bedroom to wash. To my surprise, she came right into the bathroom and insisted on taking a bath with me. Since I had soap all over my body, I didn’t want to push her to prevent an accident that would warrant me explaining to my wife and family, how her sister and I ended up in the bathroom together.
I allowed her to stay but refused to be lured into another session of lovemaking. Since she and I were the only ones in the house as my wife was away in Ghana for business, I had to sleep in the guest room that night when she insisted on spending the night with me in our bedroom. 
The following morning being a Saturday, I stayed in the room until early afternoon when the woman who cleans the house resumed for work. I came out when I heard her voice. I knew my sister-in-law will never misbehave with her around so I went back into our bedroom to take my bath as well as some change of clothes.
I didn’t bother to return to the house that day or the next. I checked into a guest house very close to my office to avoid coming into contact with my sister-in-law. I only came back home when my wife called to say she was back home.
In my determination to stay away from my sister-in-law I forgot the excuse to give my wife for leaving her sister all alone at home and opting to stay in a guest house. Since I couldn’t give her any meaningful explanations regarding my decision, my wife came to the conclusions I left to be with a woman. Her sister on the other hand is threatening to tell her sister that I raped her if I don’t continue to sleep with her because she is deeply in love with me. She said she wants to be the mother of my children as her sister will never be able to have children due to the abortions she did in the past.
I love my wife and don’t want to lose her. Despite our childlessness five years after our wedding, she remains the same woman I fell in love with. I really don’t care if she is able to give me a child or not. She and her sister are orphans, the reason I allowed her to bring her sister along when we got married.
I don’t know how to handle the issue of my wife’s suspicions without letting her in on what happened between her sister and I. I also don’t know how to handle the situation between the sister and I. Please help me.
Dear Tunde,
Much as I empathize with your helplessness at the temptations of the scene you witnessed with your sister-in-law, you should have fought against it. I appreciate it is difficult for a man to walk away from such a scene especially as his wife wasn’t around to satisfy the hunger generated by such detailed sight, self-control should have made you run from it.
That split second of indecision is what is threatening to destroy your marriage now. Nonetheless, she planned everything to entice you into her trap. Had you stopped to ponder, you would have asked why she left the main door opened while naked and watching an X-rated film, if not to compromise you.
Had you taken a second to think, it would have acted as iced water to douse the heat generated by the scene you met that day. You allowed yourself to be tempted because deep inside of you, you are attracted to her and wanted it as much as she did. Unfortunately, you cannot take back what happened between the two of you that day. It occurred and she is capitalizing on that incident to want you to continue to play her game.
Before she does greater damage to your marriage by announcing a phantom pregnancy, confess what happened that day to your wife as well as the threats her sister is making to ensure you keep dancing to her tunes. It is imperative however that she knows you are confessing because you want to and not because of her sister’s blackmails.
There is no doubt that she will not take kindly to your betrayal, but its best she hears it from you than her sister whose aim is to destroy your union. Telling her will answer the question of why you left the house to stay in a guest house until she came back from her trip. She may not believe you at first but by the time she cools off and able to think clearly, she would realize you are telling the truth because ordinarily you wouldn’t leave the house to stay in a guest house if you didn’t have reasons to.
That you didn’t hide the fact that you stayed in a guest house while she was away will eventually win the case for you.
Telling her will also take the winds off the blackmail sail of your sister-in-law. She appears to have the upper hand now because she thinks you are incapable of admitting to your wife that you had sex with her. The moment you tell the truth to your wife, her sister can no longer accuse you of rape or any other sex crime charges she might have loved to bring up against you.
She will be the one trying to defend herself against your confessions and begging her way through the mess. It will also help your wife to know the true character of her sister, especially the bit about the nature of her childlessness. In addition, it will help you cure whatever sexual attraction you may have for your sister-in-law.
Be clear that the intentions isn’t to justify what you did or cause a rift between the sisters but to safeguard your marriage from total collapse, should her sister get to your wife first.
Don’t expect her to forgive you instantly. Bear in mind that she has been betrayed by the two people closest to her. It will definitely take time and even when she says she has forgiven you, she will never be able to forget that she was betrayed by her sister. Therefore, don’t ever stop apologizing for betraying her trust no matter how difficult she becomes. Time and love will always conquer the most difficult hurdle in any marriage.
Finally, commit your marriage into the hands of God. You need Him more than ever before to emerge stronger and better from this mess.
Good luck,