I can’t activate my sisters iPhone

Hi

You'd be better asking this in the phone section. You are in the Big Sur section.

However:

Is this a recently acquired used phone? Then, it might be activation locked to the previous users apple ID, and you will need the previous user to unlock it. If the seller can't arrange this then you will need to return the phone for a refund.

https://support.apple.com/en-gb/HT201441

If she has previously used the phone successfully, but has forgotten her apple ID Password, then see here:

Forgotten apple ID Password

https://support.apple.com/en-gb/HT201487

Forgotten apple ID

https://support.apple.com/en-gb/HT201354

Apple ID recovery

https://support.apple.com/en-gb/HT204921

If you think your apple ID has been compromised

https://support.apple.com/en-gb/HT201487