"Kim is in a vulnerable position because she's very fragile and everyone is worried how Monty's death will affect her," a source tells PEOPLE

Credit: Kim Richards Instagram

As Kim Richards mourns her beloved ex-husband, those close to the former Real Housewives of Beverly Hills star’s worry how losing him might impact her sobriety.

“Kim is in a vulnerable position because she’s very fragile and everyone is worried how Monty’s death will affect her,” a source close to the family tells PEOPLE. “Everyone is hoping it won’t send her off the deep end, because she’s dealt with so many issues already.”

Richards, 51, was married to Monty Brinson from 1985-88, and they share a daughter, Brooke, 29. Brinson died of cancer this week, and he lived with the child actress toward the end of his battle.

Brinson supported his ex-wife when her alcoholism spiraled out of control again last year. (She has since completed rehab, and Kyle Richards told PEOPLE days before Brinson’s death that her sister is “in a great place” with her recovery.)

“Monty was Kim’s best friend. She can’t believe he’s really gone,” the source says. “It’s all incredibly painful. Brooke is such a sweet girl and for her to lose her father so young is just heartbreaking.”

As for Kyle and Kim’s up-and-down relationship, the source says the siblings are at peace during this difficult time.

“Kyle is supporting Kim throughout all of this,” the source explains. “No matter how strained their relationship, Kim and Kyle are sisters and family is more important than anything. Nothing else matters when a tragedy like this happens.”