(SALT LAKE CITY) -- Family fireworks are on full display in a Utah courtroom where a judge is expected to decide soon whether a 12-year-old girl should testify in her father's upcoming murder trial after finding her mother dead in a bathtub in 2007. Martin MacNeill, 58, a once-prominent doctor and former Mormon Sunday school teacher, is accused of killing his wife in April 2007 to pursue a relationship with his mistress, prosecutors say.
Investigators say MacNeill "intentionally overdosed" Michele MacNeill, 50, after she had plastic surgery, because she had approached him about his alleged affair. Martin MacNeill was arrested in September 2012 and charged with first-degree murder. He has denied the allegations.
Defense attorney Susanne Gustin argued Tuesday that MacNeill's youngest daughter, Ada, should not be allowed to take the stand. The defense says prosecutors allowed Ada's older sister, Alexis Somers, to interview her and potentially swayed her answers.
Prosecutors say there's no evidence Ada's testimony is tainted even though MacNeill's older daughters have long-believed their father is guilty of the alleged crime. MacNeill's other daughter, Rachel MacNeill, also believes her father is guilty.
"He had a plan to murder my mother, and this was just part of his plan," Somers told ABC News in 2012.
Somers and Rachel MacNeill are expected to be among the prosecution's star witnesses. Martin MacNeill's daughters and other family members have consistently been a presence in the court during pre-trial hearings, often clutching pictures of their mother as they glared at their father.
MacNeill's alleged affair with Gypsy Willis, who moved into the family's home as a nanny weeks after Michele MacNeill's death, is also on the witness list.
The judge has also allowed Martin MacNeill's fellow inmates to take the stand and jury selection is expected to take place next week in a trial that pits a patriarch against his own family.
Investigators believe MacNeill drugged and drowned his wife, who was found unconscious in the bathtub. Prosecutors believe MacNeill gave his wife a dangerous combination of Valium, Percocet and Ambien while recovering from a facelift they say he compelled her to have.
"I think he drugged my mother and drowned her," Somers said outside court in 2012. "It's been horrifying, and horrifying that we had to wait for this day so long."
MacNeill says his wife drowned in the bath tub and he dialed 911 to call for help.
Somers described to ABC News' 20/20 what Ada told her on the day she found her mother dead.
"Ada said that my dad told her to check on your mother. My dad stayed in the kitchen while she went into the bathroom and found her," Somers said.
MacNeill also served three years in a Texas federal prison for fraud and was released in July 2012.
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