I don't think Khushis biological mother has anything to do with Arnavs father, but I think the 'woh aurut' dadiji was talking about is Garima she probably worked as a domestic help at the Malik household and Arnavs father fell in love with her. I am only saying this because I found a character sketch of Garima Gupta on Star Plus website, read the highlighted bit!
Garima is Khushi and Payal's mother. She is somebody who considers self-respect and family honour topmost. Fear of infamy haunts her, as she has gone through that ordeal when she was a young woman.
http://starplus.startv.in/characters.aspx?sid=124
And secondly we all know Garima is the second wife of Sashi Gupta and step mother of payal, why did she get married to a widow with a young daughter, if this was her first marriage? I think after whatever happened between her and Arnavs father people would have started talking, questioning young Garimas character and self-respect. It would have been difficult to find a partner after all this, so her family pressurises her in to marrying Sashi Babua!
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