Why my iPhone unlocks with my sisters face AND mine????

It’s either because you and your sibling look alike or because you have shared with your sibling the passcode to your iPhone and Face ID has learned how to recognise her too.

You can learn how Face ID works and never stops to learn how to recognise you from ‘About Face ID advanced technology’: https://support.apple.com/en-us/HT208108

Change the passcode first then reset the Face ID then see if she can still unlock your iPhone. Never share your passcode with others, otherwise you can have bigger issues than your sister able to unlock the device with Face ID.

Setup Face ID again, with no one else in the room.

Create a new passcode.

Do not share your passcode with anyone, unless you want them opening your phone.

It's not unusual for close blood relatives to identify as the same person using faceid and other facial recognition systems.

Does your sister know your passcode?

Was your sister present in the room when you established Face ID?

Were you present in the room when your sister opened the device?

But, does she know your passcode?