Where does the animosity between Baby Kochamma and the twins come from?

The obvious answer is that she thinks they are sneaky and untrustworthy, and that they will steal from her.

However, I think that there are other reasons for the animosity. It only started to surface as Estha and Rahel got older. Remember the bit about Ammu, Baby and Rahel in the bathroom? They were all loving towards each other then.

But as they get older, and move back in, I think that Baby Kochamma is secretly bitter about the attention the twins start to get. It's not always even positive attention, but it is attention nontheless. I think that Baby was (at least superficially) comfortable, if not happy, with her life, and took comfort in her small material world, and she didn't want anything to interfere with that.