The world's largest Christian TV channel, the California-based Trinity Broadcasting Network, has become embroiled in a multimillion-dollar financial scandal after members of the family that founded it alleged widespread embezzlement.

The claims – by Brittany Koper, whose grandfather Paul Crouch founded TBN, and by Joseph McVeigh, another family member – describe exorbitant spending on mansions in California, Tennessee and Florida, private jets and even a $100,000 (£63,000) mobile home to house the dogs of Crouch's flamboyant wife, Janice.

The network, which claims to broadcast in every continent except the Antarctic and has 18,000 affiliates, was set up by Crouch in the 1970s and preaches a "prosperity gospel" which promises material rewards to those who give generously.

Two years ago it declared a net worth of more than $800m, although in recent years it has faced increasing financial problems. Details of the claims are contained in cases filed with the California courts by McVeigh, who says he was targeted by the network, and 26-year-old Koper, who was fired in September.

According to the lawsuit, reported in US newspapers, Paul Crouch Sr obtained a $50m luxury jet for his personal use through a "sham loan", while church funds – many of which come from donations during events like its "Praise-a-thons" – paid for the dogs' mobile home.

McVeigh's lawsuit makes the most damning allegations, claiming "unlawful and unreported income distributions to Trinity Broadcasting's directors" with "multiple jet aircraft, including a $50m Global Express luxury jet aircraft purchased for the personal use of the Crouches through a sham loan … as well as an $8m Hawker jet aircraft purchased by Trinity Broadcasting for the personal use of director Janice Crouch".

It also describes the purchase of "multiple motor vehicles, including a $100,000 motor home purchased by Trinity Broadcasting as a mobile residence for director Janice Crouch's dogs".

Directors of the network are also accused of misusing funds to cover up sex scandals, including the alleged "cover-up and destruction of evidence concerning a bloody sexual assault involving Trinity Broadcasting and affiliated Holy Land Experience employees; the cover-up of director Janice Crouch's affair with a staff member at the Holy Land Experience; the cover-up of director Paul Crouch's use of Trinity Broadcasting funds to pay for a legal settlement with Enoch Lonnie Ford (a former TBN employee who said he had a homosexual affair with [founder] Paul Crouch)".

Brittany Koper, the network's former finance director, claims she was fired after she discovered the extent of the financial wrongdoing.

Her lawsuit follows one by the church against her – later dismissed – which alleged that Koper and her husband used forged documents to embezzle funds to buy cars, jewellery and a fishing boat.

"She blew the whistle and got terminated," Koper's lawyer, Tymothy MacLeod, told the Los Angeles Times. "Brittany has done the right thing. It's admirable that someone on the inside of TBN has come forward and is revealing to the world exactly what is going on behind those closed doors."

"These large ministries, they do become family enterprises… and in many ways that can be a most precarious problem for them," David E. Harrell, a professor emeritus of American religion at Auburn University, who has written about well-known televangelists told Associated Press. "Business squabbles, if they're complicated with family squabbles, can get nasty indeed."

TBN is no stranger to outside scrutiny. In 1998, the elder Crouch secretly paid an accuser $425,000 to keep quiet about allegations of a homosexual encounter which he has consistently denied, saying he settled only to avoid a costly and embarrassing trial.

In 2000, after a five-year battle, a federal appeals court overturned a ruling by the Federal Communications Commission that found Crouch had created a "sham" minority company to get around limits on the number of TV stations he could own.

The network's lawyer has denied allegations accusing McVeigh and the Kopers of working together to steal from the ministry. He said the Crouches travel by private jet because they have had "scores of death threats, more than the president of the United States".