A concubine is any consort to a ruling member of a Great House.
Concubines were often taken by ruling members of Great Houses for companionship. This was considered an ideal practice when one wished for other Great Houses to think they were available for alliance through marriage.
Over the next few decades, Leto Atreides II inherited the throne with his sister-wife, Ghanima at his side after the death of their parents. Irulan's nephew, Prince Farad'n Corrino became Ghanima's concubine.