You mention that some of the family may have migrated to America. So there's an RChat board for the USA and another for Canada. But there's 50 US states, and the USA's population is approx 315 million. So it is a good idea to obtain some details about the ancestors first, which is why I was asking about the NAA files.
You can see the names and dates of birth for your inlaws on the online index. Your husband likely is named on your father in law's death cert, so that should also give you info about your husband's paternal grandparents, as likely your mother in law helped provide these. So let me for one moment assume your family is NSW based (only because I am NSW Centric, so I have a fair understanding of the protocols for NSW, each Australian state works slightly differently from each other) .
In NSW the general public cannot access death certs for the first 30 years. However, if you are named on the certificate (or expect that you are named on it), you are eligible to apply to purchase it from the NSW BDM. NSW BDM death certs include the names of the deceased's parents, and the names and ages of the deceased's children, and the names of the spouse, and when and where married.
Of course, these NSW BDM records also include detailed information about the deceased himself. .... so his full name, his occupation, his usual address, his age, ..... all information that the funeral director asks of the family, when organising the funeral service. It is very detailed, perhaps one of the reasons for the 30 year restricted access
Yes, of course, there can be mistakes made when providing this information, and family history buffs understand and take that into consideration.
But, we need a starting point, which is why I suggest the NAA records. Afterall, they are details given by your husband's mum and dad and checked by Australian immigraton officers.
Cheers, JM