Two of the suspects were previously charged with sex offenses involving a 3-year-old.
Police said Sheila Rumback and Scott Trumps, both of Opelousas, were arrested Wednesday after a four-month investigation.
In May, police arrested Paul Rumback of Opelousas and Ricky Lavergne of Florida on charges of molesting a 3-year-old.
They were also charged Wednesday in the case involving the 6-year-old.
All four will be sent to lineup on Thursday.
The status of the May case is indeterminate, since sex cases in St. Landry Parish are shielded from public record by district court tradition.
The same will be so of the latest arrests.
Scott Trumps was arrested for one count of Aggravated Incest. Paul Rumback was arrested for one additional count of Aggravated Incest. Ricky Lavergne was arrested on an additional count of Molestation of a Juvenile. Sheila Rumback was arrested for Accessory After the fact of Aggravated Incest and Accessory After the fact of Molestation of a Juvenile.