The Encyclopedia of World Problems and Human Potential says Genetic Sexual Attraction is:
Experienced between mothers and sons, fathers and daughters, and between more distant relatives, but most common between siblings of opposite sex who bear a close resemblance. 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. The attraction gives rise to a sense of underlying shame and guilt, together with a feeling of rejection that may prevent effective communication because the emotions are too threatening to share with anyone. This may be compounded by any sexual relationship resulting from the attraction.
Adoption rights advocates write about GSA being a frequently noted response to reunion. They say family members experiencing GSA might feel the need to touch, spend time together, talk and share.