Nuns nowadays often use their birth name.
If you have her sister’s death, was there an obituary or an announcement where the family might be mentioned?
My suggestion also, or obituaries of parents or other siblings. In the US they would usually say the nun's name and at least "of Washington, DC" for example. And they might include the letters identifying the order, etc.
And newspapers, in case she became a school principal, etc., and did something to get into the papers.