2,243. A Definite Kohein of Unknown Paternity

Hilchos Issurei Biah 20:18

Let’s say that the sons of two kohanim got mixed up or the widow of a kohein married another kohein without waiting three months after her first husband’s death with the result that we don’t know whether the child was fathered by the first husband and born on time or fathered by the second husband and born prematurely. They must therefore observe the stringencies that would apply if he was the son of both men: he must mourn for both of them and they must both mourn for him; he may not render himself unclean for them, nor they for him; he may serve in the same duty watch of kohanim as both of them but he doesn’t receive a portion. If the two men are from the same duty watch and the same father’s house, then he receives one portion.

Hilchos Issurei Biah 20:19

The above applies when the relations with both men were in the context of marriage. If one was a promiscuous relationship, the Sages ruled that the son is not considered a kohein at all because of the doubts surrounding his paternity. This is based on Numbers 25:13, “He and his descendants after him will possess it,” i.e., this only applies to descendants who can trace themselves back to him.