Why does my 2 yr old g/d act like this?

Hi I have a Grand daughter who is 2. I keep her on the weekends while her parents work. She is a great child Never into trouble never cries or throws a fit. Even when its bedtime she gets in her bed with no problems. Anytime she does act up like not picking up toys or something like that (very rare) she gets put in time out.

My son is a very very good daddy to her. and she is a Daddy girl. Her parents also put her in time out at home when she does not behave. They do work A lot but her dad is home 2 days per week plus in the late evenings. Here is the problem. Anytime Her parents walk in esp. her dad No matter what she was doing or how she was acting she instantly starts running to him to pick her up (he does and loves on her) and then cries and throws a fit the whole rest of the time they are here. Even if they bring her to someplace like for Christmas we could not enjoy ourselves for her throwing a fit and crying and throwing fits. And when they are around she acts like she doesn't know any of the rest of us.

I cant get over it she is like 2 different kids completely when with us alone and when they are around. They do put her in time out and try talking to her but nothing works. I have even tried to talk to her before they get there to tell her that i expect her to behave and that mommy and daddy like to see her smile not cry but it doesn't work. any ideas on why she does this and how to fix it? And now to top it off she has a new baby sister.