When everyone said they wanted Sims to have more interesting lives, I don't think any of us meant incest. Nor did EA intend it, so a Sims 4 patch yesterday has addressed the sudden tendency for Sims to try pursuing a romance with their relations.
A few different bugs cropped up after the romance-focused Sims 4 Lovestruck expansion and a base game update last week but the two most prominent were Sims being way too mean to each other for no particular reason and Sims deciding to develop illicit relationships with their family members. It was happening through the "Neighborhood Stories" feature which allows Sims outside your played family to have life milestones of their own like marriages, pregnancies, job changes, or accidental deaths without your direction. Nice, in theory, if you want a more lively neighborhood developing around you that you don't have to micromanage.
