I think only bensonmum was murdered. In the beginning of the movie, bensonmum is seen talking to "lionel twain" who's voice is only heard but you know it's truman capote speaking because, like james earl jones, capote had a very unique voice. Later on, bensonmum is seen talking to yetta the maid. So in a couple of separate scenes, bensonmum is interacting with both "twain" and yetta. BUT, at no point in the movie are yetta and "twain" seen together at the same time. Which means that, in my own conclusion, means that yetta and twain were the same person throughout the whole movie and bensonmum was the victim. Yetta was than able to impersonate bensonmum just the same as she impersonated twain, who, if jessica marbles is correct, died 5 years prior.
And the note about her being deaf, mute and unable to read english, that was just a big fat lie, and probably a test to see if bensonmum was truly blind.