It may just be out of old habit, since she was Smriti's Saas and that is what Purva used to call her, but she should change the way she calls her Saas from "Auntyji" to Maa or Sasumaa or Maaji or Mummyji, for as long as she is married to Mihir.
Anybody else realise this and think she should change the words she uses?