After watching yesterday's episode, I don't think Yash is Radha's son. If he were her son, why on the earth she goes to stab him? may be both Yash & Akash are twins of Gayathri & SP. She would have succeded in staeling on child from the hospital and the other would have escaped by crying or something.
This Radh could be SP's sister in law (his brother's wife or Love interest) from a low profile family of whom the family has abanodoned. may be her husband is died of some himiliation by SP and she wants to take revenge for that so she is planning all the humiliation by using his own son Akash. SP is silent about her identity because none of his family members are aware of his brother.
Jaya