I do remember abhi telling daadi he proposed tanu before marriage so im assuming she knows he was in a relationship before and maybe during recovery of pragya's accident pragya has told daadi her and abhi never had a hubby/wife relation...who knows what happened in those 20 days...
When Daadi got to know of TaBhi pregnancy, it seemed like she believed Abhi was in a regular marriage with Pragya and then fell for Tannu..And Pragya tried to convince Daadi Abhi loves Tannu, so he will be happy with her.. But she never said anything about their past. But Daadi/Daasis clearly knew Abhi loves Pragya. I don't know what they made out of the whole thing.