2 months back i poste this Topic:
http://www.india-forums.com/forum_posts.asp?TID=415941&K W=srsr
(I think Nandhini pretends that she got married again in front of Karan & Family.And tries to make Karan hate her.
She might came back to unite(close) Karan & Tanya
My guess is Ganga & Nandin are business parteners or Ganga was in touch with Nandini from last 20 years. Ganga might told her about Karan & Tanya's relationship and how much he is missing her.)