why is stopwa biased?

Why does she hate khushi who is garima's neice and not payal who is her own daughter?! Shouldn't she hate both equally? Or rather payal more?
And she hated khushi before she knew about garima. So maybe she resembles someone she knows? Shouldn't she hate payal more then?
or is the secret related to khushi's birth parents?
How is she related to payal anyway??