What is considered incest, and how it is treated, varies a great deal depending on the time and the culture. Some cultural groups consider it all right for close relatives to marry. On the other hand, other groups insist that people must leave the village, or the clan, to seek a mate. At certain Native American social events you will hear people asking each other what their clan is in order to find out if they can date.
Laws vary from country to country, religion to religion, and state to state.
The type of incest that people tend to be most concerned about is parent-child incest. This tends to be forbidden in almost all cultures, and with good reason. Sexual contact between an adult and a child is never acceptable for any reason. This is not a show of love. This is a way to ruin a child's trust of others forever.