The manner of Aunt Loretta's death is somewhat ambiguous. She suffers an injury playing tennis and is taken to hospital where she's put on a course of antibiotics. It seems that her death is caused by an adverse reaction to the medication she's given. Yet it's also suggested that she dies of AIDS. It's notable that Loretta tells Helen that her husband Roger's been sleeping around with both men and women. And given that the the story is set in the early 80s, when the AIDS epidemic first began, it's not unreasonable to conclude that Aunt Loretta may have contracted the HIV virus from her promiscuous husband. This is by no means a definitive answer to your question, but it's a plausible interpretation nonetheless.