I am sure Dev's mother will come to RP's house, now tht Dev knows about his parents he will surely ask DDG about his mother...Padma knowing that she is being cheated by her Hari might pack her bags and goes with Rohan to JhumriTalaiyya...
I think dev's mother is alive because dadaji did not mention his son's first wife to be dead or maybe he may tell today how dev ended up in this family or there also may be possibility that dev's mother would have made dadaji to promise that dev's should never be known as his grandson until the right time comes. That will be reason why he kept quite since the story how dev ended up in the family is not revealed yet'.