CHALLENGE: Can they afford to retire at ages 65 and 68 while also laying out a considerable sum to help Ms. Johnson's 76-year-old mother, who expects to need financial assistance?

BALANCE SHEET: The Johnsons have a combined annual income that averages around $140,000, about $600,000 in retirement savings (primarily mutual funds invested in large-cap U.S. stocks), $70,000 in cash and a house valued at $275,000 that's almost paid for. They also try to save about $20,000 a year combined in her simplified employee pension and his 401(k).

Overall, about two-thirds of their current investments are in equities.

ADVICE: The Johnsons are in decent financial shape, but they need to diversify their holdings, says Barry Kaplan, a certified financial planner in Atlanta. Mr. Kaplan suggests adding some midcap and small-cap stock funds to lower their risk through diversification. These have the potential to deliver better returns, helping the couple build up their assets and putting them in a stronger position to help Ms. Johnson's mother.

Of course, those sectors can also be riskier than big stocks, so Mr. Kaplan suggests adding some stability to the portfolio with bond funds, such as Vanguard Group's Short-Term Bond Index and Inflation-Protected Securities funds.

Clark Randall, a certified financial planner in Dallas, suggests that the Johnsons reduce their equity holdings to about 50% of their current investments from two-thirds. He also advises them to pursue a strategy that delivers better income than large caps—and gives them more assets to make ends meet—while offering stability. His suggested portfolio: long-term bonds, real-estate investment trusts that aren't publicly traded, and regulated hedge funds, such as the Calamos Market Neutral Fund and the Merger Fund.

As for Ms. Johnson's mother, Mr. Randall suggests that the Johnsons could buy her house by setting up a mortgage between themselves, with no lender involved. Over the next 25 years, the Johnsons would pay Ms. Johnson's mother about $9,500 annually for the place—about what they're spending now on their children in college, both of whom are expected to finish within two years.

But there's a risk here. Mr. Randall is counting on the house more than doubling in value in the next 25 years to $400,000, at which point the Johnsons could sell it and fold that money into their savings. If the house doesn't appreciate enough, the Johnsons have a slimmer chance of making their money last throughout retirement.

Both planners think the Johnsons need long-term-care insurance. "My very rough rule of thumb is that if your net worth is less than $500,000, you can't afford [the premiums]. If it's north of $3 million, you can self-insure," Mr. Kaplan says.

To cover the average cost of daily care in Kansas, Mr. Kaplan suggests buying a policy that pays at least $150 a day. It should also include "inflation protection," meaning the payments rise with inflation, along with a "shared care" rider, meaning one spouse could tap benefits left unused by the other, he says.