You might have better luck if you look up each person listed as survivors in their obituaries. Do you have a copy of your mother's obituary? If yes, compile a list of survivors and where they were living at that time.
Take the list to your local library. Most libraries provide access to Ancestry and you can lookup death info in Ancestry.
Manuela Solis died June 1978 in McAllen, Hidalgo Co., TX. Source: Social Security Death Index.
Do a Google search for the McAllen Public Library and ask if they'll copy her obituary. Successful Searching!