Harper Lee never explicitly discloses when Bob Ewell's wife died or how old Mayella was when her mother passed. The audience learns in chapter 3 that Burris Ewell does not have a mother and that her "paw's right contentious." During the Tom Robinson trial, Bob Ewell takes the witness stand and begins by testifying that he is Mayella's father before commenting that "her ma's dead." Bob does not elaborate on how or when his wife passed away. In chapter 18, Atticus cross-examines Mayella and asks how long her mother's been dead. Mayella responds by saying:

"Don't know—long time." (Lee 185)

Mayella is nineteen years old when she takes the witness stand, and the audience can infer that Burris is probably eight years old. On the first day of school, Scout is five and Burris has been repeating the first grade for three years. If Burris enrolled when he was five and failed three years, he would be eight years old. If Burris is the youngest of the eight Ewell children and Mayella is the oldest, than she would have been eleven years old when her mother had Burris. Therefore, it is possible that Mayella shared the same mother with all of her siblings.

It is also important to remember that Maycomb is a conservative Christian town, and people would certainly gossip about children born out of wedlock. Without explicit details, the audience is left to infer that all of the Ewell children shared the same mother.

To answer your question, we will refer to the text in the novel. Accordingly, the first reference to the Ewell mother is at school, when Miss Caroline asks concerned questions about Burris Ewell. Apparently, Burris has been attending the first day of first grade for three years and then staying home for the rest of the school year. One of the older students advises Miss Caroline to just mark Burris absent for the rest of the year, as that is the usual practice of teachers at the school (when it came to the Ewell children).

"But what about their parents?" asked Miss Caroline in genuine concern.

"Ain't got no mother...and their paw's right contentious."

The text states that the Ewells have lived behind the Maycomb county dump and thrived on 'county welfare money for three generations.'

The next reference to Bob Ewell's wife is during the trial when Bob takes the stand.

"Are you the father of Mayella Ewell?" was the next question.

"Well, if I ain't, I can't do nothing about it now, her ma's dead," was the answer.

A third reference to the Ewell wife and mother is made during Mayella's time on the stand.

"How long has your mother been dead?'

"Don't know-long time."

"How long did you go to school?"

"Two year- three year- don't know."

From Mayella's answer, we can conclude only one thing: her subjective answers are an unreliable guide to aid us in figuring out certain timelines. A 'long time' could be five, six, ten or more years; Mayella's lack of certainty regarding the length of time she attended school also adds to her unreliability regarding facts and events.

Either way, in answer to your question, Mayella's mother could very well have birthed all seven of Mayella's siblings before her death. Bob Ewell is also not forthcoming as to the exact time of his wife's death. He merely states that Mayella's mother is dead. At this point, we don't have enough information to conclude whether Mayella and her siblings actually shared the same mother, although the possibility is certainly evident.