What is incest legally and what punishment applies?
The Mississippi general prohibition against incest is that
Persons being within the degrees within which marriages are prohibited by law to be incestuous and void, or persons who are prohibited from marrying by reason of blood and between whom marriage is declared to be unlawful and void, who shall cohabit, or live together as husband and wife, or be guilty of a single act of adultery or fornication, upon conviction, shall be punished by imprisonment in the penitentiary for a term not exceeding ten (10) years.
The degree of relationship declared to prohibit marriage is defined as
(1) The son shall not marry his grandmother, his mother, or his stepmother; the brother his sister; the father his daughter, or his legally adopted daughter, or his grand-daughter; the son shall not marry the daughter of his father begotten of his stepmother, or his aunt, being his father's or mother's sister, nor shall the children of brother or sister, or brothers and sisters intermarry being first cousins by blood. The father shall not marry his son's widow; a man shall not marry his wife's daughter, or his wife's daughter's daughter, or his wife's son's daughter, or the daughter of his brother or sister; and the like prohibition shall extend to females in the same degrees. All marriages prohibited by this subsection are incestuous and void.
In the US, incest laws are at the state level, not the federal level.