Q: Say if I had sexual relations with a half sibling who is in their 40's would it still be illegal?

A: §21-885. Incest

Persons who, being within the degrees of consanguinity within which marriages are by the laws of the state declared incestuous and void, intermarry with each other, or commit adultery or fornication with each other, shall be guilty of a felony punishable by imprisonment in the custody of the Department of Corrections not exceeding ten (10) years. Except for persons sentenced to life or life without parole, any person sentenced to imprisonment for two (2) years or more for a violation of this subsection shall be required to serve a term of post-imprisonment supervision pursuant to subparagraph f of paragraph 1 of subsection A of Section 991a of Title 22 of the Oklahoma Statutes under conditions determined by the Department of Corrections. The jury shall be advised that the mandatory post-imprisonment supervision shall be in addition to the actual imprisonment.

If you can't marry them, you can't have sex with them.

Relationships that are considered too close thus constitute incestuous relationships in Oklahoma:

Any ancestors and descendants (parent/child, grandparent/grandchild, etc);

Stepparent and stepchild;

Uncles and nieces (except if relationship is only by marriage);

Aunts and nephews (except if relationship is only by marriage);

Brothers and sisters (including half siblings); and

First cousins.

These relationships are declared to be incestuous, illegal, and void. They are expressly prohibited. Oklahoma recognizes a marriage between first cousins if the marriage happens in a state where it’s legal.