What happened to Mrs. Tendo?
It’s never said how Mrs. Tendo passed away, but she did so when Akane was quite young. It was probably from natural causes though. Miss Tendo is never mentioned in the manga until Akane has a dream about her (Vol 22 Part 2). The Tendos, Ranma and Genma go to her grave to pay their respects to her on the anniversary of her death. As indicated by Akane’s memories of her mother, she was a very kind and nurturing woman.