View Message

Since her parents were Jewish, the most "obvious" explanation is surely that they used this as an alternative for Sara or Sarah.
In her book "The History of Christian Names" (google it), page 48, the 19th century British pioneer of name meanings and usage Charlotte Mary Yonge gives Zara as an "eastern" form of Sarah, meaning "princess".
Here in Australia, there was a well-known public figure called Zara (Dame Zara Bate, whose second husband,Harold Holt, was prime minister of this country in the 1960s. She was born in 1909, so it may be that the name was mildly fashionable in English-speaking countries at that time.
If you enter the name into the Google Books search option you may turn up something literary, dating to that period.
There is, for instance, a long work, "Zara ... a poem of the sea", dating to 1833, published in London, so the name was certainly known by the time your ancestor was born.