The Sims 4 currently contains an inconvenient bug where some characters in the game want an incestuous relationship. Electronic Arts is aware of the bug and is currently working on a fix.
The Sims players reported via the official forum from the game that Sims need to have a romantic relationship with family members. For example, a player reports that a character in the game wants her son as a friend; for obvious reasons not a normal wish of anyone, especially not in the usually kid-friendly Sims game.
Some Sims’ glaring need is probably caused by the new wants system, small goals, and Sims’ wishes as a replacement for the current whims. Players are encouraged to serve Sims in their wants as points can be earned for doing so. This new feature is part of an update leading up to the High School Years expansion for The Sims 4. The same update also appears to have caused another bug that causes Sims to age unnaturally quickly or even die suddenly.
Developer SimGuruNick lets know that EA has managed to reproduce the wants issue and is currently working on a fix. The bug that causes rapid aging is also being treated.