We learn from Tom Robinson's testimony that Mayella had given him a strong impression that her father was forcing her to have incestuous relations with him. The Ewells are the quintessential representatives of what Southerners call "white trash." They are lazy, mean, immoral, vicious, and a burden to the community. Although the possible father-daughter incest is not spelled out in so many words, any adult reader would guess what was going on between Bob Ewell and his daughter. The story is told through the point of view of Scout, a very young girl, and there is a good possibility that she didn't understand some of the things that were being said or implied on the witness stand. Bob Ewell's seething hatred for Atticus is at least partly attributable to the fact that Atticus has exposed him as being even an even more immoral character than most of the community already knew or suspected. He abuses his daughter Mayella both physically and sexually. Atticus probably didn't try to dig deeper into their relationship because that would not necessarily help his client Tom Robinson and might even be rejected as irrelevant and immaterial. The only way Atticus could prove incest--if he had decided to try to do so--would have been to get Mayella to testify to it; but she refuses to do so, probably out of deathly fear of her father's reprisal. Everything about Bob Ewell makes the reader think that incest would not trouble his conscience any more than any of his other misdeeds.
There are many things about the Ewells, that we learn in To Kill a Mockingbird. Scout tells us that the Ewell children come to school on the first day, then they don't come again the rest of the year, and no one questions it.
Atticus tells Scout that the Ewells are the disgrace of Maycomb county and none of them had ever worked an honest days work in their life. Mr. Ewell spent most of his welfare check on booze and most of the landowners turned a blind eye to his poaching, because his children were starving. Mr. Ewell is a widower and has eight children. His daughter is the one who accuses Tom Robinson of rape. Mr. Ewell is a mean man and wants to take justice into his own hands.
When they go to court, he tells elaborate lies about Tom Robinson, and Atticus knows they are lies, so he has to prove it. Scout is in the courtroom and is amazed at how Mr. Ewell has acted on the stand.
" As Judge Taylor banged his gavel, Mr. Ewell was sitting smugly in the witness chair, surveying his handiwork. With one phrase he had turned happy picknickers into a sulky, tense, murmuring crowd, being slowly hypnotized by the gavel taps lessening in intensity until the only sound in the courtroom was a dim pink-pink-pink: the judge might have been rapping the bench with a pencil."
Throughout the entire story, we know that the Ewells are not the nicest of people. We see by the end, that they can be deadly.