Whitney Houston's Family -- Fortune is 'Too Much Too Soon' for Bobbi Kristina

Whitney Houston's family wants to put the singer's fortune on lockdown to stop Bobbi Kristina from blowing her inheritance ... and to protect BK from financial leeches.

Cissy Houston and Pat Houston, Whitney's mom and sister-in-law, filed a request this week to modify Whitney's will. Specifically, they want to change the distribution schedule of the estate ... estimated to be worth at least $20 million.

Sources close to the family tell TMZ ... Cissy and Pat filed the documents because they think the current will gives BK "too much too soon" -- and they don't think she has any need for that kinda cash now.

We're told the family also wants to "protect [Bobbi Kristina] from herself" ... because she's vulnerable. In the documents, they claim 19-year-old Bobbi "is a highly visible target for those who would exert undue influence over her inheritance."

Bobbi Kristina, Whitney's sole heir, is currently scheduled to get her inheritance in three chunks -- 10% when she turns 21, just under 20% at age 25 ... and the balance when she's 30.

In their petition, Pat and Cissy don't mention what specific changes they are seeking to the current payment schedule -- but they say Whitney's intent was "to provide long-tern financial security and protection" for her daughter.