Resetting watch without old Apple ID

Hi,

So a little background:

In 2017, my father was gifted an Apple Watch series-3 by his company for Employee of the year. Unfortunately, we didn't use Apple products at the time. Only my sister-- who was in a university in my home country-- had an iphone 5s. When she came for vacation, we paired the watch with the iphone, compared the desgin and stuff with our Galaxy watch and then turned it off and re-packed it. Soon after, my sister bought S7 edge and passed the iphone 5s to me and I factory reset it. Created my own Apple ID.

Its July,2019. We almost completely forgot that we had paired it. My father gifted it to his brother-in-law last week. Turns out that we need old apple ID to pair it (duh). But we have completely forgotten both the apple ID and the password. We've since been trying our absolute best to try and remember atleast the ID but to no avail.

Any way to get it to work?