My sister is using my old phone, but for some reason our contacts are synced

So about a week ago, my sister received my old iPhone 6s because I got a newer one with more storage. And she had just broken her own phone, the one she had prior to receiving mine. On her SIM card she had saved all of her contacts, and before importing them she had logged in with her own Apple-ID and then imported them. But for some reason, my Apple-ID changed name to her's, and I got all of her contacts added to mine.

Then I deleted the contacts that were added, and changed the name on my Apple-ID back to my actual name. But for some reason the same changes were applied to her phone. Now I'm not sure what is going on, I've tried telling her to log out in settings, log back in with her own Apple-ID and then import the contacts again, but now for some reason the contacts won't be imported from the SIM card. She still doesn't have her contacts, and our accounts still seem to be somehow synced, since whenever we change the name of our own contact, it changes the other's as well.

I can't think of any solution to this, since I've never experienced something like this with my phone anymore, but I really hope someone can help.