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.