Final answer:
The family structure that best describes Helen's household with her grandchildren, whose mother is in jail, is 'skipped generation,' a subtype of an extended family.
Step-by-step explanation:
Helen's household with her grandchildren, where the grandchildren's mother is absent due to incarceration, is best described by the term skipped generation family. This is a specific type of extended family structure, which often sees grandparents taking on the role of primary caregivers when the biological or adoptive parents are unable to do so. The extended family can include multiple generations and is not limited to the traditional nuclear family of parents and their children. However, in Helen's case, because the middle generation (the children's parents) is missing and she is the primary caregiver, it is more accurately described as a skipped generation household.