T-Boz says her family is taking legal action against the police department for the shooting death of her mentally ill cousin ... and she's behind them, 100%.

We got the TLC singer Wednesday night at Avalon Hollywood, and asked if she'd heard back from cops in Peoria, Illinois about her cousin Eddie Russell Jr.'s death last week. He was gunned down by officers in what they call a standoff that turned life-threatening.

T-Boz says she and her fam haven't heard a peep, but plan to file a wrongful death lawsuit against the cops. As we reported, officers fired 17-20 rounds at Eddie, who was on the run after allegedly attempting to rob a bank.

She explained where she believes cops bungled the case -- and strongly feels Eddie's punishment didn't fit his crime.