Aunt Saffronia is one of the first actors we meet at Austenland, and her appearance highlights the absurdity of the whole place. She not only comes armed with her own backstory, she also comes with a whole made-up life for Jane. "I am your Aunt Saffronia, although you do not remember me as I haven't kissed your cheeks since you were two" (4.15). And Saffronia goes on and on and on. We're not sure how Jane keeps it all straight.
Other than leading the nightly dinner march and being one of the first actors Jane sees break character—"the actress bit her nails, but Aunt Saffronia did not" (4.23)—she doesn't have much bearing on the plot. However, it is nice to see her care for Jane a bit after Jane is assaulted by Sir John. Aunt Saffronia might not love Jane like a daughter, but at least she loves her like a pretend niece.