Would anyone be interested in a long term brother/sister line?
The idea is that they were orphaned when they were younger and separated in the system. They meet each other years later, and realize they have an instant connection, and start dating. It can either get to the point where they are married and trying to have kids, or are engaged and looking to get married, and somehow they find out the truth. So the line would still have to involve dealing with them coming to terms with their actual relationship, if they should stay together and eventually finds it adds more to their kinks in bed to know they are brother and sister.