DNA matches Q

[Merry]

Previously I said on another thread I did wonder if my great-grandfather was really my great-grandfather or whether he was duped into marrying his housekeeper because she was expecting a baby (my gran).

Looking at my DNA matches I see Ancestry has picked up that I am 'extremely' likely to be a second cousin to an individual (Shared DNA: 246cM across 14 segments). This person's grandmother is sister to my grandmother and both those woman are children of the housekeeper and maybe of my supposed great-grandfather.

My Q is, if the two grandmothers/sisters had different fathers would I be able to tell by the degree of DNA match, or are there too many variables to make any assumptions?