Saloni had seen Tara suffering and craving for her baby. She also knows the reason why Tara was not able to conceive. She had seen Tara being humiliated during shubhra's god bharai function. She knows how much grief she had to go through for getting Advitiya back into the family.
Also the writers brought out another sentiment in here. In joint families, children are loved by everyone. Sometimes the children may spend more time with their uncles, aunts,granny etc.. rather than with their own parents.As a person who has been brought up in a joint family, i can give the testimony to this. Even in the show, Saloni has been shown giving affection to advitiya more than taking care of her unborn baby.