Molly Martens Corbett showed "explosive anger" towards her husband's children, court documents have claimed.
Martens and her father are both accused of Limerick man Jason Corbett's murder.
Father-of-two Jason (39) died from head injuries at his home in North Carolina last August.
In freshly released court documents, Mr Corbett's sister claimed that Martens (32) had obsessive and volatile behaviour.
One alleged incident cited involved a claim Ms Martens held Jack Corbett's head in a sink, allowing water to pour into his mouth, after the boy had splashed her during a family outing.
Mr Corbett's sister Tracey Lynch accused Martens of being obsessed with getting the children, Jack (11) and Sarah (9), to call her "mom".
The children were born to Mr Corbett's first wife Mags, who died when they were infants.
Mrs Lynch also alleged that Martens had displayed "volatile behaviours" and "explosive anger" toward them.
The allegations were all denied by Ms Martens.
She lost her bid to become guardian of the children after her husband's killing last August. Mrs Lynch now cares for the children with husband David.
Court filings also reveal Ms Martens has signalled her intention to mount a fresh bid for custody.
She filed a notice of appeal last month. The move came after an initial appeal failed on December 8.
Earlier this week, Ms Martens and her father, former FBI agent Thomas Martens (65), were charged with the second degree murder and voluntary manslaughter of Mr Corbett.
A police report from the incident on August 2 said her father told a 911 dispatcher he had struck his son-in-law with a baseball bat during a row.
Martens and her father claim self defence and are out on bail.
However, the claims made against her during the custody battle are now likely to be revisited when their trial gets under way later this year or early in 2017.