WHITNEY HOUSTON’s family is in panic mode over news that the singer’s troubled daughter Bobbi Kristina plans on marrying her “adopted brother,” say sources.

The 20-year-old, called Krissi by pals, has publicly announced her engagement to Nick Gordon, who was raised by the late Grammy-winning singer. And that has her 79-year-old grandmother Cissy Houston and other family members fearing for Whitney’s fortune, the sources add.

“Cissy’s worst fears are coming true,” a Houston family insider told The ENQUIRER. “She never trusted Nick and says he’s an opportunist with no job. She’s convinced that he’s only marrying Krissi to get a piece of Whitney’s $20 million estate.”

The family keeps a close eye on the youngster, who’s been plagued by emotional problems since her mom’s tragic drug overdose death on Feb. 11, 2012.

Krissi’s marriage plans so upset Cissy, she called a family meeting to discuss the disturbing turn of events.

Sources say Cissy is determined to convince her granddaughter that she’s making a big mistake. If that doesn’t work, she’ll instruct her attorney to draw up an airtight prenuptial agreement for Nick to sign.

Krissi, whose father is R&B singer Bobby Brown, shocked her family and viewers of last year’s Lifetime reality series “The Hous­tons: On Our Own” when she revealed her engagement to 24-year-old Nick. Few took the announcement seriously, but on July 10, Krissi confirmed her wedding plans in a defiant Facebook message, declaring: “People need to seriously stop judging my relationship.”