Well I definitely don't believe that Kamla loves Kalpi. When there are conditions for being good to your child, that is not love, that is conditional love which is not true love.
Kamla has unconditional love for Pakhi, she loves her no matter what, even over her own children, she defies even her husband for her.
Kamla cannot see pass Pakhi, because that is the child that she truly loves. Whether it is hard to digest or not, it is true.
Vitthal said it today, and she never denied it once. A normal Mother who loves her child more, would have shouted in protest, that she does not love Pakhi more than Kalpi. She would have said she loved them equally at least.
Kamla is the Mother we hear about, and glad we don't have.
I had already decided who Kamla was when she saved Pakhi from drowning and never looked back for Kalpi. She never even comforted Kalpi after that. I think I have seen other Kamla's of the world, a maid who worships the boss's daughter, till they believe this child is their own. I believe that people like Kamla feel so unworthy as maids, that they see their own children as unworthy.
Kamla puts Pakhi on a pedestal not because she just gave her milk, but because she gave her boss's daughter her milk and that is an honour for her.
Kamla feels honoured to be part of Pakhi's life and considered her Mother because she feels less than the Kapoors.
Every time Neetu makes a fake attempt to be nice to Kamla she is so flattered and willing to forget all her wrongs, even against her own daughter because Kamla feels honoured to be part of their lives.
Kalpi's love means nothing to her Mother because it is coming from a maid's daughter, but Pakhi's love is coming from a girl of a rich family, and that is what makes Kamla feel good and flattered that a maid could be loved by her boss. She is so gentle and caring to Pakhi the little princess and she is aggressive and unloving to her 'Poor' daughter Kalpi.
this is my analysis in attempt to answer the question of why Kamla behaves this way.