Here is what I think..and will be not surprised if similar story unfolds.
She might be wife (married without Dadu's approval) or girl friend of NK's older brother who is passed away.. or left her...while she was pregnant.
Knowing that ..Dadu brought her in CH, got Nirmala ...Sandy's friend married to NK to give Sandy's child NK's name... who is Kunal.
that's why she is abusing Karan ..hurting Karan's and Nirmala's ..self confidence... to keep Herself and Kunal above all in CH!!
I noticed...she was not worried for Nirmala missing Kunal's marriage..when it came to Karan's marrage...Sandy mentioned that she should call Nirmala atleast...She is Mom after all.
Now after these two sisters entry - her importance is upside down - she is not able to digest it...