I am a US citizen and I filed I-130 & I-485 for my mother to gain her permanent resident. She's in the United States as well as my little sister who is under 18. Would my little sister get her status upgrade as well because of her age? Or do I have to file separately for her?
You will have to file separately. Your mother can also file once she gets her green card. But note that your sister will not have any status in USA during the multiple years her green card application would be pending.