My 2 yr old's behavior toward me(mom) is steadily worsening. She is kind & loving w/ others. Any ideas?

She was very obedient until about 18 months old. Nothing out of the ordinary happened then, and my other daughter wasn't born until she was 30 months. I have tried any & every bit of advice I've been given. NOTHING works. Her father, and grandparents do not have a problem with her doing what she is told to do, only me. I have tried to use their methods, but to no avail. I am the primary caregiver in our home. I have tried to take some 'mommy-time' & have her grandparents watch her, but she refuses to spend the night or even a day with her dad away from me. When she goes home with someone, they end up bringing her back shortly because she won't stop crying for me. She won't even go in my parents home without me, because she's afraid I'll leave without her. It's not getting any better now that she has a 2 month old sister. She is very loving & caring with her sis, not acting jealous or resentful of her at all. I'm at a loss.I want her to be happy & healthy but she doesnt seem to be.