how do I get my wife to do incest with her brother
For most of the people I’ve interviewed, consanguinamory has been about a deep and strong connection with someone they love, someone they want to be with the rest of their lives. However, there are people who have various fantasies, turn-ons, or fetishes when it comes to consanguineous sex. Someone looking for ideas about how to get their wife to have sex with her brother might find the idea itself stimulating. People who want to see (or at least hear about) their partner being with another person aren't all that rare. That fantasy is a common one, in contrast to people who would never want to think about their partner being with someone else. Also, it isn’t rare for someone to find it arousing to have a threesome with partner and the partner’s sibling of the same gender (for example, how many men fantasize about having sisters/twins?) It is a little less common to want your partner to be with their different-gender sibling.
There might be different reasons someone would want that. Maybe they have their own experiences with siblings or want to, and so they either want their partner to experience the same thing (very strong sense of compersion) or to be open to the idea of them being with their own sibling. After all, if the wife is having sex with her brother then the person who did the search would, in theory, have an easier time negotiating with the wife about being with their own sibling.
Maybe the person who wants this wants the wife’s brother for a threesome experience or ongoing triad, and figures the easiest way to make that happen is to get the wife to start things up with her brother? There’s a chance the person simply wants a threesome with a(nother) male involved and figures the safest person to involve would be the wife’s brother.
So the WHY could be different things, and make no mistake, fantasies are a very different matter than actually doing something.
As far as HOW to make it happen, well, that can be complicated.
Much of what was written here applies, but what was written there was about one person who already had an interest pursuing another person (often, whose interest or aversion is unknown), not one person trying to get two other people together. For this to be possible, neither the wife nor her brother can have a strong aversion to being with each other, which a person may have due to what is described as the Westermarck effect, or some personal moral principle, or just plain not being attracted enough. (I'm assuming the brother isn't in a closed relationship.) The "easiest" way to get these two together is to blindfold/earplug both of them and make it a surprise, but I strongly recommend against that unless they have both, separately, expressed a strong desire for each other but are both too uptight to make a move.
The phrasing of the search does not give us a solid indication of how her brother might feel about this, but it appears the searcher doesn't know how his wife feels about it. It is possible the brother has expressed interest and the searcher is trying to help a brother-in-law out. In that case, the searcher does have an advantage: presumably, the searcher knows how to seduce this woman, since the searcher is married to the woman. As such, tips can be given to the brother.
Something that might help is knowing their history together. If they ever explored or experimented at all before, that can be useful. It is also possible to find out their thoughts about nonmonogamy or polyamory specifically without revealing the desire for it to involve a sibling. But both topics can be raised by pointing out that in some polygynous and many polyandrous marriages, it isn't uncommon for siblings to share the same spouse, and seeing that their opinions are about that.
If either one or both of their desires and boundaries are unknown, then separate discussions, which might have to start out vague or with hypotheticals or generalities, would be a good idea. If the brother is completely averse to sex talk that involves mentions of his sister, then there's the end of it right there. But if the the searcher starts talking about sex and plays up the prowess of the wife, and her brother is eager or at least open to further discussion about it, that's a good sign, though not necessarily a guarantee he'd be interested in experiencing that for himself.
The searcher can try role-playing with the wife. If she isn't turned off by calling her "sister" during sex, that might be an opening to explore her openness to the fantasy and and the reality.
As already said, much of what was written here can be adapted. Getting away together, talking about sex, creating erotic situations, watching genre erotica together, role playing... anything that gets them closer together in the romantic or sexual sense can help. If there is contact dancing involved, the spouse can ask to take a break and ask the brother to take over. Earlier I mentioned the brother not being in a closed relationship. If the brother does have a partner and the relationship is not closed, the partner can help create the situation, provided the partner is on board, which adds to the complications. If the partner is willing, it could be a matter of setting up double dates and maneuvering the situation just the right way.
Since every situation is different, it is impossible, in an entry like this, to completely answer the question for any one person who has it. I did, though, want to wrap up with an additional word of caution. If everything works out and the goal is achieved, consanguinamory can provide a very powerful bond. What if the searcher becomes the "third wheel?" That is a possibility, and nobody who thinks they want their spouse to get with a sibling should neglect to consider that. It's a complicated situation, because it involves the considerations that come with nonmonogamy and the considerations that come with consanguinamory.