Not entirely sure, but sounds like might be a carrier locking problem. That is, the phone is locked to the carrier that originally supplied it, and is different to the SIM you have installed. The other carrier may be willing to unlock it, for a fee ranging from peanuts to ££.
Various websites claim to show whether a phone is locked to a carrier (find them with google) or, if you visit an Apple store, I expect they’d be able to tell you.
It is also possible your sister has put the phone into ‘lost mode’, using the Find My iPhone App or using iCloud. That is used when a phone is stolen, and effectively bricks the device, until cancelled by the legitimate owner. I don’t think that’s likely, as it’s hard to do by accident, I don’t think a phone in ‘lost mode’ would get as far as yours does.
Sounds like there is also a separate issue, that may or may not matter, that your sister’s iCloud account has become locked owing to too many incorrect login attempts. Pretty sure if she just waits 24 hours she’ll get another chance, else she’d have to try password recovery...
https://support.apple.com/en-gb/HT204106