Kathleen McQueen - Theresa's biological mum and sister to head of the family Myra (Nicole Barber-Lane) - will be played by Alison Burrows.
She will make her first appearance in one episode of this year's Hollyoaks Later run, which is being headed up by former Hollyoaks producer Bryan Kirkwood.
While Theresa is in London on a mission to pursue her modelling dream, she decides to visit her mother, who is currently spending time at Her Majesty's pleasure in prison.
Michaela (Hollie-Jay Bowes) helps her cousin keep the visit a secret, but warns her that Carmel (Gemma Merna) and Jacqui (Claire Cooper) highly disapprove of their aunt.
When she eventually comes face-to-face with her mum, Theresa is delighted by her intentions to turn her life around when she is released. However, the youngster's dream of a reunion crumbles when Kathleen admits that she is moving to Spain with her boyfriend.
A Hollyoaks source told DS: "Theresa's devastated when he mum gives her the brush off but she's soon back with her tail between her legs towards the end of the year in time for Christmas, though."
They added: "Just how will Theresa react to her mum wanting to be a part of her life again? And how will Myra cope with her sister's sudden reappearance?"
Burrows's Later episode is due to transmit on Wednesday, September 30. She will then descend on the village in a full-time role during the main Channel 4 show in December.
Her previous screen credits include Emmerdale, Coronation Street and Waterloo Road.
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