She does know. She heard Sai's full rant in the guise of comedy about how her ex husband is on honeymoon with his current wife and how she is showing off. She knew that they were working together. She even consoles her that she should stop denying that she's not affected by seeing the person she used to love now with another and that its not wrong to feel sad and cry. She invites Sai to unburden and that isnt shown in detail but Sahiba knows that Sai has kids with her ex husband. She didn't know that one kid was not hers though. Because she asks if the other child was also hers but she clarifies that his name is a coincidence and he is somebody else's Vinu. So Sahiba doesn't know that Sai's ex-husband has another kid with his 2nd wife. But she knows he was on honeymoon with 2nd wife so how the heck is she stupid enough to pray that Sai should get Vinu back and be a happy family with her ex husband?! Moreover when Sai says the kid isnt in her life anymore because dur ho gaya, the first instinct is to assume he's dead na.. Not that he's actually "lost" . Thats much less likely na. No one will crassly say ha mera baccha mar gaya.
They made Sahiba look like an idiot.