I need to unlock my sister apply watch

If you know her Apple ID and password, and it had not been deleted then erase it as follows

If you forgot your Apple Watch passcode - Apple Support

pair it with an iphone, and enter those Apple credentials

If you don't know her Apple ID and password then I would go to an Apple Store with

Proof of your identity

A copy of the or probate or some other proof that you inherited it.

Certified copy of the death certificate

See How to request access to a deceased family member's Apple account