Two answers for you! Or one answer for you!
The parents relate to the 4-10 axis or the IC-MC axis of the chart. Given the basic concept expressed within classical Astrology (from 100 years ago or more) the 4th house would be the Mother of the person (1st house of self). If this is so, the Mother's mother would be the 4th house of the 4th house -- or the 7th house. Let's explain derivative houses to some degree: The 4th house is the house that represents the mother -- therefore the fourth house is the 1st house of the 4th house. The fifth house is the 2nd house of the 4th house. The sixth house is the 3rd house of the 4th house. And, the seventh house is the 4th house of the 4th house -- the Mother's mother, or Grandmother.
Now, if you subscribe to the 10th house as representing the Mother, we can apply the same logic. If the 10th is the Mother, the 10th house of the 10th house is the Grandmother. The answer is, of course, the 7th house which represents the Grandmother -- just as if we favored the 4th house as representing the Mother.
So, you have two answers -- but both answers are the same. The seventh house represents the Grandmother. Dave