Need help, my sister is being scammed by a caregiver. Any advice?

Call the police or Adult Protective Services.

If you think she's being scammed, call your local Adult Protection Services. Elder abuse doesn't only mean physical abuse. Financial abuse is just as prevalent. Have some evidence of your suspicions.

What is happening that makes you think she is being financially abused?
Are you (or anyone) her durable PoA?
Where does she live? Where do you live?
Do you have free access to her?
How old is your sister? Why does she require a caregiver?
Do you have other siblings who you can consult with in this issue?

Can you provide more detail?