We get letters from people at all stages of their lives and relationships. Some are freshly into adulthood, some are middle-aged some are older. Sometimes they are thinking back fondly on good times of the distant past, sometimes they are talking about something going on at the present time, and others are considering the future, like these siblings.
A comment was left here by Anonymous...
The question was specifically about ethics.
I generally don't think it is ethical to mislead someone into believing you are sexually monogamous with them when you are not.This goes whether your other relationship(s) is/are consanguineous or not. Someone might want to make the case that if someone falls in love with you and is satisfied with you in every way while you are secretly having sex with your sister all along, there is no need for them to know. But I don't think that is ideal, and as I noted, it can result in serious trouble.
There's more than one way to ethically handle this situation while the consanguineous freedom to marry is still denied and such lovers are still denied protection. Here are some (and they are made on the assumption you will not be entering into marriages arranged by your parents)...
1. Don't marry or otherwise get into a relationship in which the other person expects monogamy. It is also important to keep in mind that someone who wants an open marriage or a polyamorous relationship could still be someone who would not accept that their metamour is their lover's sibling. Unfortunately, in many places, people are pressured strongly to enter into (heterosexual, nonconsanguineous) marriages.
2. If marriages are allowed between in-laws where you live, if you could find another brother-sister couple that would be in agreement, you could marry the sister in that couple and your sister could marry the brother.
3. Provided you want a polyamorous relationship, marry someone who is supportive of your polyamory and your relationship with your sister.
4. It would also be possible to marry an understanding lesbian who wants to stay closeted. You can help keep each others' secrets.
Your sister could do something similar to 3 or 4 if you did one of those.
It is not so easy to find people willing to assist with 2, 3, and 4, but it is possible. Hmm, perhaps there is a matchmaking operation just waiting to be developed? Until then, or until people don't have to hide their love anymore, you might find willing people on discussion forums.
I wonder if readers have other ethical solutions? If you do, leave a comment below.
It's ridiculous that consenting adults should feel it necessary to hide, but that's still the case, unfortunately. These lovers should be free to marry each other without fear.
See Has Your Partner Experienced Consanguinamory?