Cuz when sindura's mum was still alive, bade thakur was always working and she felt deprived and started dying and told sin that bade thakur doesn't love them. The when she died bade thakur was not so busy and got remarried to Uma and sin started hating all of the (uma and sagar) cuz she felt bade thakur luved them more than he loved her...
I'm not exactly sure if it's true or not but that what i heard... 😆