“The No. 1 complaint is ‘Why aren’t my siblings helping me?’” said Amy Goyer, the caregiving expert at AARP and author of “Juggling Work and Caregiving.” “There has to be someone who takes the lead and organizes others to do things, but it’s hard for the ones who feel that they’re doing all the work.”

According to a 2001 report in the journal Conflict Resolution Quarterly, nearly 40 percent of adult children who cared for a parent said they experienced major conflict with a sibling. That conflict could be “over the amount of care, or money, or who should be making decisions, or just deep-rooted sibling rivalry over who does Mom or Dad love best,” said the report’s author, Deborah B. Gentry, a professor emeritus at Illinois State University.

“Most of the time siblings want what’s best for the parents. They just look at it in a different way,” said Susanne Terry, a mediator in Danville, Vt. “Our goal is to help them figure out what their common interests are, so they can work together to find solutions.”

Unlike traditional psychotherapy, which can take years to unearth entrenched problems, mediation is more focused on the present. The mediator does not advocate the viewpoint of any one person; rather, he or she acts as an impartial third party, helping resolve conflicts in ways that are acceptable to everyone.

Before Janet E. Mitchell, a lawyer and mediator in Fort Wayne, Ind., first meets with families, she has them complete a 25-question online inventory about how they respond to conflict. Sessions typically last two to three hours and cost $150 an hour. (The McMahans paid about $100 an hour for three two-hour sessions.)

“I help them make family decisions they can all live with,” Ms. Mitchell said. “I help to bring out their better selves so they can focus on the parent or the continuation of the family.”

Sally Smith, an environmental artist in Westport, N.Y., remembers how her father, Clyde H. Smith, and aunt, Nancy Whiton, benefited from mediation. Thirteen years separated them in age, and they had been rivals “ever since Nancy was born,” Ms. Smith said. When it came to making decisions about their elderly mother, Hilda Smith, they constantly undermined each other. Ms. Whiton, who provided all the physical care, wanted her mother to go into a nursing facility, whereas Mr. Smith, who had power of attorney, wanted her to remain in her home for as long as possible.