Romantic attraction is actually a composite of emotional attraction (admiration, mutually expressed as friendship) and sexual attraction (lust, when manifested alone).
A familial bond such as those between parent and child, or those between siblings, can satisfy the emotional component of romantic attraction.
However, this emotional bond usually comes from one (the child in parent/child) or both (the siblings) parties having spent their formative years around the other, and this also produces a sexual aversion towards those they have spent their formative years with, this effect is called the "westermarck effect", named after the psychologist who discovered it <jew>. (meaning that every child being fucked in CP by their parent finds the act repulsive on the instinctual level).
The reason why Genetic Sexual Attraction exists is because the children have been separated from their parents or siblings during their formative years, because of this, they have not developed the sexual aversion of the westermarck effect, and another component ensures that a sexual attraction is, in fact, very likely.
From an evolutionary perspective, finding a partner that has a high likelihood of being genetically compatible, so finding people that resemble family members attractive would be encouraged, of course, actually fucking your family has a higher chance of producing defective offspring, but the westermarck effect would ensure that this wouldn't be likely to occur in our paleolithic surroundings (chances were that even if you were separated from your family members during your formative years,, it meant that you would never see them again).
That is, until the modern world came along, now parents were being reunited with their long-lost children, and long-separated siblings were reuniting with each other, without the westermarck effect, the genetic sexual attraction kicked in as if they were unrelated but genetically similar potential partners, instead of actually being related to each other.
In parent-child situations, this is only an explanation of the child's perspective, the parent's is more freudian, the separation usually leaves the parent with some measure of guilt, and a need for emotional closeness, which, when opportunity presents itself, can manifest in a desire for physical (read: sexual) closeness.