As we all know Bindu easily gets manipulated that was the reason why both Chanakya and Dharma were waiting for uchit samay to reveal about Ashok being his son coz normally would they had told others ke wajah se Bindu would still have doubted coz as I sayhe easilycones into others trap... that was the reason why they wanted it to come out on perfect time so that he would never have any doubts related to Ashok being his son.
I think in this case they don't want Ashok telling the sacchai and later if Siamak and his sathis suppose create a situation that what Ashok said was false...this can lead to Bindu never trusting Ashok again and also Siamak will be get aware. She basically wants to asure their chances of getting nothing wrong.