Fans are so acutely aware of the issue that the only way to work into the story that they are mother and daughter would be to slip it in in an underplayed way. Perhaps the two of them had developed a shared mutual silent understanding over the years, and then a new event causes it to be spoken. If DS had continued into the 70s, social changes might have made it more acceptable to admit to the circumstances.
The alternative would be a storyline in which there is some clever twist on it all that would actually surprise and stun us, in a satisfying way. Fans are unlikely to come up with anything that good, I'd think, though I don't know DS fanfic.