My brother's 35 and says our parents' inheritance is his whole retirement plan. How can I talk some sense into him?

An estimated 20% of U.S. adults expect to receive an inheritance from their loved ones, and more than half of those expecting to become heirs describe their future inheritance as "critical" or "highly critical" to their long-term financial security.

Since 31% of U.S. adults anticipate leaving an inheritance, some of those individuals who believe they will inherit may find their expectations are met. However, this won't happen for everyone. That can be a problem when someone is counting on an influx of cash that never comes.

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Let's say, for example, that Rodney is 35 years old and is counting on his parents to leave him a large sum. He's told his sister Josephine that he's not saving for retirement because he's confident his inheritance will be enough.

However, Josephine is concerned, and rightly so. Now, she's wondering what she can do to convince Rodney to see sense and start shoring up his own future instead of sitting around waiting for Mom and Dad to die. And she's right to worry, as the experts are on her side.

You can't count on an inheritance to come

Rodney's first big problem is that he's putting all his eggs in one basket, and there's no guarantee that betting on Mom and Dad to provide an inheritance will pan out.

"I've seen many situations where parents leave less than the children expected, or set up a trust directing everything to charity and nothing to the kids," Adam Spiegelman, founder and wealth advisor at Spiegelman Wealth Management, told Moneywise.

"A client I work with is giving their entire estate away to charity, and their adult children, who are in their fifties, have no idea. That's Mom and Dad's right and privilege, but it illustrates the point: you can't count on anything," Spiegelman said.

Rodney's parents may decide not to give him the money, or something outside their control may happen. For example, around 7 in 10 people turning 65 will need long-term care at some point during their lives, which can cost upwards of $100,000 per year . Rodney's parents may be forced to spend their money on a nursing home before he inherits a dime, even if they promised he'd be getting the cash.