Ask. Find out everything you can, while the people and their memories are still around. You may not get a single coherent story — is there ever a single coherent view of anything, outside fiction? — but the alternative is speculation and regret and one day, having to tell your granddaughter “I don’t know, because I never asked, because …”
There’s a very good reason why you keep that recipe. You just don’t know what it is yet.