What a great question!
Generally, this is an issue you would find in the index of grammar books under "collective nouns." When we use words like family, couple, church, company, or other collective nouns, choosing what verb to use is not clear.
The basic guideline is this: if the group is acting as a single unit, then the noun acts singularly. If the word is not acting as a separate unit, then use the plural.
In the above example, my grandmother is correct in saying, "Harlan's family was devout Methodist and his father's family was Quaker."
On the other hand, "this couple was the fraternal grandparents of Harlan," I would suggest were. The couple is not exactly acting separately in this sentence, but the subject complement is plural (grandparents), and if you turned the sentence around to say, "the fraternal grandparents were this couple," the "were" seems clear.
Grammar Girl has a great blog that talks about this: Grammar Girl
Last cool thing for now. My amazing husband got me this! 1898 grammar is better than today's?