Frank Cascio and his parents and siblings are protesting an effort by Jackson's estate to keep their claims behind closed doors.
The family alleges Jackson groomed, manipulated and molested them for decades, from the late 1980s until his death in 2009.
On Wednesday, they appeared in a Beverly Hills courtroom seeking to void a "purported settlement" with Jackson's estate that they describe as "an unlawful agreement to silence victims of childhood sexual abuse".
A judge heard arguments but declined to issue an immediate ruling on the estate's petition to force the family into confidential arbitration.
Marty Singer, a lawyer for the estate, told the court that the Cascio family members signed an initial deal with the estate in January 2020, then later re-negotiated it for "significantly more money upfront", and now are seeking to file a public lawsuit that would violate the arbitration and confidentiality clauses of the original pact.
"We categorically dispute these claims," Singer told the court, referring to the claims Jackson subjected all five Cascio children to sexual abuse.
"The reason this case is going forward is because there was an extortion demand of $213 million (£158 million) last summer."
Mark Geragos, lawyer for the Cascios, told the court the family had felt coerced into signing the agreements.
"The rushed process was intended to, and did, in fact, take advantage of the Cascio siblings' shock and trauma upon realising this had happened to all of them, unbeknownst to each other and contrary to what they had been told," Geragos wrote in a filing last October.
"During this vulnerable time and before the Cascios could fully process what had happened to them, the estate exploited their confusion and vulnerability by pressuring them into an unfavourable agreement, misrepresenting both the nature of their rights and the consequences of refusal."
Geragos previously represented Jackson when the pop star was under criminal investigation for child molestation in 2003.
The Thriller singer was charged and later acquitted at a trial in 2005.
