Sometimes relationship terms are also used as terms of affection, even if there is no biological relationship. Take care when a letter, diary, or a relative refers to someone as an "aunt" or an "uncle." The use of the term may have been done out of respect and not necessarily indicate a biological relationship.
Of course, you may gain some clues or insight by researching this person, but if you find no biological connection between the individual and your family be open to the possibility that "Grandma" was[n't] really "Grandma" after all.
Thanks to reader N for this suggested tip.
And thanks to anonymous who notified me of the typo! The correction has been made in red.