Hi Michelle - whilst this is a public forum it doesn't really operate in that way as it's primarily a communication channel between posters and HMLR. Less of a discussion/chat forum as a result.
From a purely registration perspective you need to start from how the property is registered -
Search for land and property information - GOV.UK
It reads as if both in laws were joint owners and that the legal ownership has not been updated/transferred
If so then you will need Letters of Administration for the in law who out-lived the other
Once you have confirmed the legal ownership and obtained letters of administration to enable a sale to take place