For the most part we can only speculate about Mayella's relationship with her father, but some clues are revealed from the testimony given during the trial. We know that Mayella's mother is dead and that she now serves as the surrogate mother for the other seven children. Mayella has no time for a social life or to make friends since she spends every waking hour keeping an eye out on the kids while Bob is out hunting, prowling or getting drunk. We can assume that he mistreats Mayella when he's been drinking since Mayella seems to acknowledge it in court, and Mayella no doubt goes along with Bob's story out of fear for her father. When Atticus asks if Bob is "easy to get along with," Mayella responds that
"He does tollable, 'cept when--"
"... Except when nothin'," said Mayella. "I said he does tollable."
"... Except when he's drinking?" asked Atticus so agently that Mayella nodded. (Chapter 18)
Tom's testimony reveals two important bits of information. First, when Mayella is making her move on Tom, she tells him that she's
"... never kissed a grown man before... what her papa do don't count." (Chapter 19)
This suggests the possibility of impropriety between Bob and Mayella and, perhaps, even an incestuous relationship. Later, according to Tom, Bob threatens her: "... you goddam whore, I'll kill ya"--proof that Bob is the man who actually beat his daughter. Atticus seems to think it's probably not the first time Bob has beaten his kids. While explaining to Jem about why Bob accosted Atticus, he explains that
"... if spitting in my face and threatening me saved Mayella Ewell one extra beating, that's something I'll gladly take... I'd rather it be me than that houseful of children out there." (Chapter 23)
Bob obviously has no respect for women--black or white, woman or child. He stalks Helen Robinson, "crooning foul words" to her; and later tries to kill Scout out of his hatred for Atticus. Mayella is probably lucky that Bob does not carry out his threat to kill her as well.