Best solution for multiple Apple ID's following a death

This is a bit of a curly one. My mother in law and recently passed away and she tended to manage all of the IT side of the relationship. I am currently setting up a new laptop for my father and have just discovered that the majority of apps that they used to use on their home computer are all under my mother-in-law's Apple ID. I am trying to figure out how to share those apps so that we can download them onto this new laptop, but every time I try to set up family sharing and go to accept the invites from the email on the laptop it goes straight to settings and it's like it recognises it as my father's invitation to himself and won't add the family member to the family.

For reference, they had an email account each for their respective Apple IDs, and then for some odd reason also had a third Gmail account, but I can't figure out why. Unfortunately, this is what happens when old people do technology in my experience, but I am just trying to make it as straightforward as possible for my father because he is not gifted in the technology department and doesn't have a lot of interest in becoming so.

Is there a more straightforward way of doing this, given the fact that my mother-in-law's Apple ID is largely useless now other than for the purposes of accessing all these apps. Is there anyway to notify of a death, for example, and have all the purchases, etc. transferred over to my father's Apple ID?

Thank you in advance for any help or advice.