What incident in the story was an actual incident taken from the authors' life?
While it is believed that the concept for the sledding accident was taken from a newspaper article, the true sense of autobiography is in the emotional development of Ethan Frome and the moral questions he struggles with. Wharton was not happy in her marriage, as her husband was on the controlling side and kept a mistress, on Wharton's income. While Wharton ultimately ended the marriage with divorce, this was not an action approved of in her society (also evident in her novel The Age of Inncocence). Wharton would have had to struggle with many of the same issues concerning how society would view her and interpret her actions, as Ethan did.
I think an additional perspective for this question is that since so much of what Wharton wrote was easily exemplifying the life she really knew, Ethan Frome stands out that much more being a novel set so far away from the world she knew, which afforded her some measure of freedom in, at the very least, alluding to the problems she had in her life.
It's certainly not a word for word account, especially if you consider that she's using a male's perspective as the lead character, but I think that provides yet more distance from the pain that she was really feeling in her life.
To the best of my knowledge, Ethan Frome is not strictly biographical. Edith Wharton was in an unhappy often controlling marriage, but her solution was divorce. Additionally, she did own a house in a small New England town much like Starkfield,however, she did not visit there often. As far as I know, there is no actual incident depicted.