Sister-in-law passed I received her Apple Watch. We know the pass code to erase data. Just not the password lock information. How do we move forward

My sister in law recently passed, and I have been passed down her Apple Watch. We were able to thankfully know the pass code to get into her phone and erase it but now we can’t pair it with my phone without knowing the activation lock, and Apple, ID and password, and nobody in the family seems to know. How do I move forward with this to be able to pair it with my phone. Thank you.