Gabriela Mistral did not only write poetry. She also wrote prose, and two of her works revealed her admiration for the Catholic Saints, Santa Catalina de Siena and Santa Teresa de Avila - or in English, Saint Catharine (not the saint of "catherine wheel" fame) and Saint Teresa.
In an article entitled "Siena", an extract of which was published in in the newspaper El Mercurio in December 1924, Mistral wrote a brief biographical portrait of Santa Catalina, telling "the beautiful and terrible episodes of her life". The extract begins with the lines:
She was almost a politician, this Saint, a bit of a stateswoman, as we
would say today. She is less of a mystic than our saint from Castile.
And her charity sinks into me, like a scarce water...
It is interesting to note that "our saint from Castile" can only be the other saint mentioned above, Santa Teresa de Avila (also sometimes called Santa Teresa de Jesús).
In 1924-25 Mistral visited Spain, and wrote several articles about her experiences. In an article entitled Castilla she detailed an imaginary encounter with Santa Teresa, who showed her around the Castilian countryside and recounted events from her life:
She says, I will show you my Castile, so that you can understand it...
I know your people and there is blood of my people sown in the valley
of Chile. She looks at me with her big eyes, and I know her by her
naturalness, and by the tone with which she wrote some brave letters
to Philip II.
-You are "the wanderer," I tell her; the Spanish still call you "the founder" and the pedants "the crazy woman with the love of Christ."
Mistral clearly felt a close personal and spiritual link to these women. As far as I can see she did not write about any of the other saints of these names, and so I believe they are strong candidates for the saints referenced in the final line of Vieja.