Maybe she is feeling unhappy because she knows Mom is home with sister and she isn't.

I had a 4 year old who did this for her final 3 months here. I had her since she was 14 months. This girl was just unhappy too. The parents were curious why but didn't seem concerned. I tried everything to help her and so did my husband who would sit with her and give her special attention and try to talk about what was bothering her. She left for school in this condition.

Sometimes I think these kids might be depression prone. I would just try to be supportive and give her positive feedback when she is not crying. My dcg was always quite happy to lay in bed too if I had let her. I get your frustration though as it has a bad effect on the mood in the daycare.