iPhone locked by unknown Apple ID

we are not apple

This is a user helping users with tech questions forum, we are not apple and apple do not read your posts here

about activation lock then

activation lock is not a thing on the iphone it's a theft protection on apples servers

it needs to be removed How to remove Activation Lock - Apple Support

if you don't know your appleID password If you have forgotten your Apple ID password – Apple Support (UK)

then you will need the assistance of apple

Activation Lock - Support (apple.com)

and or

Apple Support

And or

Contact - Official Apple Support

and or

Genius Bar Reservation and Apple Support Options - Apple

That was a scam which usually involves a ransom.

Notify the police.

Do not pay the ransom

Call Apple support https://getsupport.apple.com

We are not Apple Support and I think you will struggle to get it removed

Unfortunately, there's no help to be had. If it's been placed in lost mode by the scammer, you may as well toss it in the recycle bin. Apple won't remove the activation lock or a device in lost mode.

You can contact the police, but the odds of them being able to do anything about it are virtually non-existent.

Re: “She previously signed in with another person’s Apple ID because he told her to do so in order to download an app.”

Never sign into a stranger’s Apple ID, no matter what goodies they promise you, or what sob story they tell.

As you and your sister are now finding out, this is a good way to wind up with your iPhone or iPad being turned into a useless brick after scammers take it hostage.

As others have said, you've lost ownership of the phone and it's a lost cause. We see this almost on a daily basis in Russia. It's time for your sister to buy a new phone.

And not to heap on more grief, but you have posted a photo with personal information, which is NEVER a good idea to do on a public forum.