Her obsession with Bhao could not have developed just like that. It had to have been lifelong...or a long time in the making.
It would not surprise me if she had something to do with Seema's betrayal of Dutta. E.g. I could totally see Kala putting ideas into Seema's head about how pretty Seema is, how lucky they are to have her as the girl Bhao has chosen, how much Seema loves their Bhao, despite all the hardships. And how much she, as his eldest sister, appreciates the sacrifice Seema is making for love...little money, no future prospects. Perhaps, Kala even goes so far as to dramatize how she and her family have had to struggle. Seema, being a spoiled and greedy girl due in part to her awareness of her looks, was probably made to think twice about Dutta.
Or Kala would have brought Seema to the car repair shop to "visit Dutta", but moreso to catch the owner's eye. Hence, she probably purposefully designed the whole affair.
I bet you we are in for a lot of surprises with Kala's actions in the past...and I think the CVs can have a field day with the possibilities.