Janet Jackson was verbally abused by her estranged husband, Wissam Al Mana, particularly while she was pregnant with their son ... this according to Janet's older brother, Randy Jackson.

Randy claims Wissam called Janet a "bitch" every day while she was expecting. Randy added, "It came on later in the relationship, verbal abuse and being [made to feel like] a prisoner in her own home."

He went on to tell PEOPLE Janet's been breaking down into tears backstage during her recently resumed world tour ... because "the abuse is still going on" through harassment related to the custody battle over Eissa.