Possible Clues:

• Ozzie or Harriet, to Gunnar and Matthew
• Older relative
• Spoiler of a child's child
• Pregnant Dora, with nothing to lose, bothered one of the family members
• Step-up generator?
• Frequent spoiler
• Toddler spoiler, perhaps
• One of four, higher up the tree?
• Progenitor of M = X, perhaps