Here is Oklahoma's on-point statute:
Reading it, you can see that marriage is illegal when one party is under the age of 16, unless the marriage is authorized by the court, where the unmarried female is pregnant or has given birth to a child outside wedlock, and only where notice and an opportunity to be heard has been given to certain interested parties. So if the couple is already married, but has not followed this procedure, the marriage is unlawful. If the couple is not yet married, this is the procedure they would have to follow to get a valid marriage license.
However, your question leaves me unsure if the couple is married yet or not. And regardless, the deprived juvenile statutes still apply. So if the mother has allowed a 21 year old man to live in the house and have sex with her 15 year old daughter, she has committed abuse or neglect, under the definitinos given by Oklahoma's deprived juvenile statute.
A deprived juvenile case would begin here with a referral to the Leflore County Department of Human Services, who would investigate the home and recommend action by the District Attorney's office, if any is warranted.