The analyses of a text #2

The narrator recalls his adolescence.
His grand aunt came to London to meet her grand nephew the narrator.
She says to him about her worry about donkeys treading on her front yard.
Miss Murdstone is of his stepfather's ill-tempered elder sister.
Janet is the only maid of his aunt.

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"There was one this afternoon at four o'clock. A cold feeling came over me from head to foot, and I know it was donkey!"
I tried to comfort her on this point, but she rejected consolation.
"It was a donkey," said my aunt, "and it was the one with the stumpy tail which that Murdering sister of a woman rode, when she came to my house." This had been, ever since, the only name my aunt knew for Miss Murdstone. "If there is any donkey in Dover whose audacity it is harder to me to bear than another's, that," said my aunt, striking the table, is the animal!"
Janet ventured to suggest that my aunt might be disturbing herself unnecessarily, and that she believed the donkey in question was then engaged in the sand-and-gravel line of business, and was not available for purpose of trespass. But my aunt wouldn't hear of it.
[David Copperfield by Charles Dickens]
1. It is "that Murdering sister of a woman," not "~ women."
I'd like to know what "that Murdering sister of a woman" means
2. I'd like to know the animal refers to Miss Murdstone's donkey.
3. And I'd like to know what "the sand-and-gravel line of business" means.
Thank you in advance for your help.