Definition from Wiktionary, the free dictionary
stepgrandmother (plural stepgrandmothers)
- The stepmother of one's mother or father.
- The mother of one's stepmother or stepfather.
2009 January 21, Jodi Kantor, “In First Family, a Nation’s Many Faces”:
- The president’s elderly stepgrandmother brought him an oxtail fly whisk, a mark of power at home in Kenya.