"I'm sure [Jamie] thinks if only she could go back and change things she would. But, she can't – there's no point regretting the past," says Jim Gibson. "No one in the family wants her to think that way. What's done is done."
Jessica, who is suing Lowe for sexual harassment, was introduced to the actor by Jamie, who had worked for Lowe and his wife Sheryl as a nanny in 2000 for about a year, then later became their assistant.
Jamie left to get married but wanted to stop working for the couple anyway, Jim Gibson says.
"She was happy to get out of there. She couldn't take it anymore. Nothing really bad happened like it did with Jessie, but Jamie was overworked and couldn't stand it," he says.
He adds that although Jessica and Jamie have different mothers, "They don't consider each other half-sisters but full, true sisters – one family. She is concerned for Jessica and is supporting her now and tries not to dwell on the past. We've all pulled together as a family to help Jessie."
Lowe has denied the allegations and accuses Jessica of being behind an extortion plot.
For more on the case, including interviews with Gibson's family plus Lowe's friends and former employees, pick up the latest PEOPLE, on newsstands Friday