"かのじょはジョンさんのおばあさんです。"

Translation: She is Mr. John's grandmother.

[zpiper]

The alternative answer said "Mr. John" but we wouldn't use Mr. with a given name.

[OKAMOTO_Yusuke]

I quite agree, honorifics would not be used with a given name. While I found there was Mr. or Ms. John as a surname.

https://en.wikipedia.org/wiki/John_(surname)

It would be recommended to us a name which is exclusively used as a surname, e.g. Smith, Brown, Robertson etc., to avoid misunderstanding of learners.

[jmbrill81]

See, this is a problem - in this sentence, the system wants "Mr. John," however, in another, reverse sentence, it asks for just "John," but marks you incorrect if you leave off the "San."

[MichaelMom]

I cannot, for the life of me, distinguish between Japanese aunt or grandmother.

[HumanLaw]

Lol
I remember it by thinking that the word is longer depending on age.
おばさん obasan Aunt
おばあさん obaasan Grandmother

[BattleProgrammer]

is it ok to use "彼女" in this case? wouldn't "この女の人" be more respectfully?

[OKAMOTO_Yusuke]

Yes, you can use 彼女 in this case, thus 彼女はジョンさんのおばあさんです is equally correct. この女の人 is neither respectful nor derogatory, just neutral. This may be literally translated as ”this woman/lady".

If you want to say it more respectfully, use こちらの女の方 or こちらの女性の方. Generally 方 (kata) is an honorific expression of 人.