My family *is* or My family *are*? [duplicate]

In American English, collective nouns are almost always treated as singular. In British English, it often depends on whether the speaker/writer sees the noun as a unit or as individuals. That seems to defeat the purpose of collective nouns, but that is how it is.

http://www.usingenglish.com/forum/ask-teacher/3053-my-family.html

It depends on whether you could slot the word 'members' into the sentence or not.

"His family [members] are all doctors" works well enough, so you can use are.

"His family [members] is one of the oldest in the county" doesn't really work so you can't use are.

Then there is the 'one' in your example, which kind of forces it to be singular, but even "His family [members] are the oldest in the county" is a bit odd, unless the family members are all 110+.