There is a deep meaning there. You see, the only person Ishwari feeds Halwa with her own hand is Dev. She doesn't even feed her daughters Halwa with her own hand. But today she fed Sona, why? Because according to her Sona is carrying little Dev ergo, she is feeding little Dev Halwa through Sona, the carrier/vessel. Gotta start the Halwa addiction early. I can bet you that this Halwa feeding won't ever happen if a. She finds out about infertility or b. After Sona gives birth and the baby is out of her (if she is really pregnant).
Dev is shocked to see his mother treating Sona better than she treats her daughters whom she treats worse than himself. But I wonder how much shocked he will be if he finds out that it was only "chaar din ki chandni".