Here's a tip. If someone you are in a relationship with is doing things you are not willing or able to accept you can 1) ask them to stop; or 2) end or change the relationship so that the other person's behavior is no longer a problem for you. Keep in mind, when you ask them to stop, they might say "no," either in words or by saying "OK" but then continuing to do whatever it is that was bothering you, maybe after a temporary stop. So, even if you try number 1, you might still have to end or change the relationship, as painful or as much of a hassle as that may be.
In this case, it doesn't really matter if the siblings are consanguinamorous or not. Even if there is no sex involved, the fiance's behavior is not acceptable to the woman who raised the concern, and it isn't likely to change to be satisfactory.
As to whether or not there is actually anything sexual going on, we've addressed people in the (ex)bride-to-be's position before.
Let's look at how Reich responded...
Level of queasiness thus far: that weird gnocchi scene between Sofia Coppola and Andy Garcia in The Godfather Part III.OK, so the idea disgusts her.
Nononononononononono. V.C Andrews launched the thousand miniseries about this story, and they all do not end well. They end in pneumonia and homicide.Reality is often different than fiction.
Over and over again, we see that people want to portray any consanguineous sex between siblings as only something someone mentally disturbed, homicidal, or hideous would do. But the reality is different. There are many siblings out there who've experimented, or had ongoing arrangements, or were completely in love who are well adjusted, kind, and attractive. That this woman had a problematic relationship with her fiance, who wasn't treating her the way he should have, doesn't mean that there is no such thing as a good consanguinamorous relationship between siblings, no matter who is disgusted by the idea. If more people accepted that consenting adults should be free to have their relationships, fewer people would be struggling to accept themselves and they'd be far less likely to try to make unwitting beards out of others.