Communication with caregivers. Any advice?

If she has a care manager, then that person is in charge of the staff. So yes, as their boss she can tell them no talking about their client's care. IMO, if you want info on your sister, you talk to the care manager. Is your sister competent? If so, no problem is asking her but I have a feeling Dementia is involved. If so, who set up a care manager? I would say that person is in charge.

Brothers and sisters seem to think they have certain rights when it comes to a sibling. You don't, a child is next of kin and trumps a sibling.

I agree with others who point out that it may be because you are not her PoA or Medical Representative. They are just following the law. You being a sister doesn't give you any special status.

Are you her medical POA? Do you need a HIPAA release?

".. told the caregivers not to talk with us"
To clarify.. to keep on with their job & not stop to socialise & chat to family when they visit? Or to not disclose your sister's personal &/or health information to family members?

Yes it is legal, there's likely an approved list of people they are allowed to share information with.