Your idea of a foursome within the Family is not so, outlandish as it seems. There are many cases in Joint Families where a woman is shared by the brothers especially in rural areas. Often, due to accommodation constraints in cities, where a labourer shares his tenements with 10 other men, it is not possible to keep his wife with him and so he sends her to his native place to live with his brothers etc.
I want to analyse the reasons for your decision, before I comment on the positives or drawbacks of the idea.
First, it is surprising that the suggestion came from your wife. 99% of the times it would be the man who makes the first play. Did your wife know her Sister’s fiancé, before the proposal? Were they ever in contact before?
You talk about having a smooth and enjoyable threesome. A Foursome is a different kettle of fish. It is not the same as having a threesome. Your wife has told you that her sister’s fiancé was aware of the threesome. You have not spoken to the fiancé in this regard. It would be very awkward to talk about this topic later on. You need to have a conversation with him and gauge his views. I have a sneaking feeling that he has had sex with your wife and since you have had sex with his girlfriend, the implication is that all are fine with the idea.
Who initiated the idea that they will stay with you and your wife? Do you not have any say in the matter? Staying together will be a major irritant in the future. How do you decide each family’s contribution to the expenses? Any displeasure in discussing finances will be reflected in the sexual equation. It would be a better idea if they stayed separately and then visited whenever sex plans are made.
Another issue is pregnancy. Do you rush to get a DNA Test done to decide on the paternity? A DNA Test is complicated and is not easily done. If a DNA test is not done there will always be a sneaking doubt about whose child it is. If a DNA test is done, it will destroy your relationship, which has to be built on trust. After the child is born, property and hereditary issues will crop up. Are you prepared for these?
Jealousy is a factor that will come into play only when experienced. Will you all make love together or will you move into different rooms after exchanging your partners? It is not so easy to watch your spouse being made love to by another person. As you have rightly said, once may be an exception, but doing this all your life, is never going to be easy. And what you experienced is only a threesome.
What happens if your wife falls in love with her sister’s fiancé or you fall in love with your sister-in-law. I know you will say it cannot happen. My friend, Love is a fickle thing. I ask you what happens if and when it does?
There are positives too. There is a great bonding and wish to sacrifice for your extended Family. Sex will be mind blowing. But if all this were true, history and Society would have encouraged such relationships. They don’t. There must be a reason.
Think wisely in what you do. Best of Luck.