Amy Barksdale was born on May 11, 1958, Most of the fighting between Helen and Rita went on while Amy was in elementary school. If Amy was reading "weird Russian novels" at this time, she must have been (and still be) a genius.
Perhaps after high school, Helen and Rita came home for short visits, during which time Amy locked herself in her room to have an uninterrupted reading. Amy grew into her teenage years with no one else in the house but her parents, who again did not pay much attention to her. What kind of life she lived until she graduated high school can only be imagined.
Richard Lobinske was married to Rita for the first time in December 1974 or January 1975. Helen and Jake were married in June 1975. Amy would have been 16-17 then. A year or two later, given the information in Jake of Hearts, Jake's father died.
When asked about the death of Amy, Helen, and Rita's father, Glenn Eichler thought he would have passed away "when Helen was about thirty," which would be around 1980. Amy would then have been twenty-two.
Mr. Eichler said in his interviews that he thought the Barksdales were from Virginia, a Southern state with a cosmopolitan atmosphere. Amy says in Aunt Nauseam that she drove "a few hours" to get to the Morgendorffer home, and the wedding for Rita's daughter Erin was not a long drive for the Morgendorffers.
All three sisters and their mother probably live within a 200-mile wide circle, not terribly far from each other. Virginia, Pennsylvania, Delaware, and Maryland suggest themselves as possible home states.
In I Don't, Amy was last in contact with Rita before the death of her boyfriend Roger—she had no contact after that death and is unaware it happened. She can identify Daria and Quinn on sight, but is unsure how old she is; she hasn't spoken to Helen and Jake in a long enough time to 'joke' that she's amazed they're still married.
Amy Barksdale is believed to have developed a special relationship with Daria in I Don't and Through a Lens Darkly, offering her advice and moral support. In certain ways, she appeared to be a grown-up version of Daria herself (see their mirrored images in I Don't). This has been taken to the extreme in fanfiction, leading to situations in which Amy rides in like the cavalry to rescue Daria from bad situations. (Amy even says, "The cavalry has arrived!" in Aunt Nauseam.)
It becomes obvious from what she says that she was the forgotten baby of the Barksdales, always playing third fiddle against Rita's attractiveness and Helen's overachieving. She was largely ignored by her sisters, who constantly and ostentatiously struggled for attention from their parents. ("You didn't notice my sisters are so busy competing with each other that I don't even register on their radar?" she asks Daria in I Don't.) Amy's mother, and possibly her father as well, strongly favored Rita; she also pressured the girls to be more like Helen, who had become an overachiever.
The situation led, in Amy's own words to "years of bitterness and resentment" between all three siblings. Nauseam reveals that while she resents Rita being the favorite, she also resented Helen for showing them up and holds her in contempt for "nursing [her] childhood grudge" with Rita "well into adulthood." It's also Helen that gets the sharpest barb in I Don't—Amy's first words to Jake are to commend him for staying married to her—and the automatically and correctly assuming that Helen started an argument because she hasn't gotten over Rita getting a better car from their father.