Deiondra Sanders -- Deion's 19-year-old daughter from a previous relationship -- just BLASTED her step-mom on her Twitter account ... claiming Pilar Sanders has been a TERRIBLE wife to her famous father.

Before she unleashed, Deiondra apologized to her father -- claiming Deion "is going to be very upset at me 4 he always takes the higher route."

Then ... she GOES OFF -- claiming Pilar straight up LIED when she claimed she found out Deion filed for divorce on TMZ.com.

Deiondra writes, "#HowYouDidntKnow but yo boxes have been packed for weeks now."

She adds, "HowYoTopPriorityBeen yo marriage and yo kids when u flying out to see other n**gas ... yeah we know."

Deiondra added, "All u had to do was be a supportive wife. Only thing u ever cared about doing with my dad is reality shows."

"Pilar stop tryna play the victim. Maybe if u would of actually loved my dad and not loved the life he gave you we would be here today."

Pilar's lawyer, Larry Friedman, tells TMZ ... "Marriage is a relationship between two adults and it doesn't involve engaging this kind of faceless accusations made by his child."