Bev Tyler was Jacob and Winnie Tyler’s daughter. Her father abandoned the family in June 1977, even though he knew Winnie was pregnant.
Her mother gave birth to twins. Needing money, Winnie decided to sell one of them to the Jerichos. The four-year old Bev secretly followed and saw the transaction, which took place on Christmas Eve 1977. However, Winnie was so ashamed of what she had done that she never spent the money, staying in the Quadrant and raising Bev and the twin she kept, Gabriel.
In 1987, she was killed by an N-Form activated by Gabriel’s draining the life from his twin, Steven. (Damaged Goods)
Another account suggests that she didn’t die and she and Gabriel were left in the care of their new adoptive parents, David Daniels and Harry Harvey, by the Seventh Doctor. (Damaged Goods)