Ashok in his quest to hurt Raman had claimed Ruhi to be his kid and made Shagun characterless.
Ishita in her quest to not be away from the kid decides to make Roshini characterless.
Her saying that she is doing all this as she doesn't want the baby to be separated from the mother is waste as she herself is a step mom. If this was so whatever it is she should never have taken custody of Ruhi.and Adi as whatever it is , kids should not be taken away from their mother, how could she be cruel then.
Staying with Bhallas is only going to benefit the baby as then it will get it's father's name , his right and love of whole family. She could have kept a condition that Roshini will have to stay with the baby but denying Roshini to go to Bhalla house completely is out of selfish reasons and not for benefit of Rossini.