Have you ever played the game Siblings or Dating? There's a whole instagram account dedicated to this and I always get the answer wrong.
According to researchers, we are more attracted to people who resemble our opposite-sex parents or even resemble ourselves.
There is also research that suggests marrying a 3rd or 4th cousin is optimal for reproductions. In Iceland, the study discovered that marriages between distance cousins actually produced more children.
Source: YourTango