Apparently it has a name, Dom: Genetic Sexual Attraction.
Genetic sexual attraction (GSA) was a term first coined in the 1980s by Barbara Gonyo, who founded Truth Seekers in Adoption, a Chicago-based support group for adoptees and their newfound relatives. The Encyclopaedia of World Problems and Human Potential says GSA is "particularly noted in the case of adopted children who are subsequently reunited with the biological parents or siblings of the opposite sex [who bear a close resemblance], seemingly because the normal bonding mechanism has been disrupted. It takes the form of an overpowering, almost electrical grip of emotion, associated with an inability to keep away from the other person and an almost primordial sense of having belonged together all their lives [and may lead to a sexual relationship]." The UK and US saw an emergence of GSA cases in the mid-1970s when adoption laws were abated. Adoptees had easier access to adoption records, which led to an increase in post-adoption reunions.
More—and squickier—info at www.geneticsexualattraction.com.