Bobbi Kristina Brown's substance-abuse problems are "out of control," a source close to the 18-year-old daughter of the late Whitney Houston told

The teen's mother and father, recording artist Bobby Brown, both battled addictions to drugs and alcohol.

Houston died Feb. 11 at age 48.

Her death is still under investigation, but a deadly combination of prescription drugs and alcohol is suspected as the cause of her death.

Bobbi Kristina was hospitalized twice for anxiety since her mother died. said Houston's mother, Cissy, and cousin, Dionne Warwick, are concerned about Bobbi Kristina's drug use and urging her to check into rehab to clean up.

"Whitney's passing is sending Bobbi off the deep end," one insider told the celebrity news Web site, adding the family is "shocked" by how serious her problems are. "Bobbi's problems are out of control."