When siblings are left to figure this out on their own, many of them often do not know where to start, and if there is a past dispute hidden away, it can complicate things, said Amy Castoro , a family wealth coach and president and chief executive of the Williams Group.

“Sibling resentment can go all the way back to ‘Dad didn’t go to my games, but he did yours,’” she said. “And the kids often have different interpretations of each other than a parent or grandparent might realize. So if one son is designated as the executor of a parent’s estate, the other son or daughter are looking at him saying, ‘That guy cheated at Monopoly our whole life. Why would I trust this guy now?’”

There may also have been an unequal lifestyle growing up. “The youngest kid is driving the latest BMW the parents bought for her, and the older one had to buy his own first car,” Ms. Castoro said. “They take that out on each other. These things fester. Wealth becomes a magnifier.”

Those background stories must be dealt with in a productive way, not a psychological way, she said. “I encourage families to have these estate planning conversations early on, while Mom and Dad are on this side of the daisies.”

She added that siblings “must care more about each other than they do about the money.”

Most siblings do not understand how to have conversations about money, said Thomas Lloyd, who now specializes in working on this issue with his clients. “The biggest obstacle for me was how do I relate with my brother, who is not savvy when it comes to money discussions and not comfortable, and has felt like an outsider looking in.”

Finally, last year, the brothers had a breakthrough conversation, which has made working on the foundation personally rewarding for both of them. Thomas asked for input from Stacy before he had to set an agenda for a fall meeting with the fund’s investment managers.

Stacy then started asking questions that his older brother had never realized he was wondering about. “It was a way of venting,” Thomas said.