Muriel's Wedding

Muriel Heslop (Toni Collette) is a high school dropout with no job, no prospects and no boyfriend. She uses fantasies of her wedding as a coping mechanism and also strongly matches the diagnostic criteria for a cluster-b personality disorder (possibly Borderline Personality Disorder or Histrionic Personality Disorder). In high School she was shy and socially awkward, but managed somehow to have the "in group" accept her hanging around them. Muriel attends the wedding of one her high school associates in an inappropriate leopard print dress. After witnessing the groom having sex with one of the bridesmaids, Muriel is arrested because by some strange coincidence the store detective from where she shop-lifted was also at the wedding.After this episode, her former friends (Tania, Nicole and Janine) shun Muriel because they see her as an overweight, directionless no-hoper, while continuing to plan a holiday to Hibiscus Island.
Muriel's chain-smoking, unambitious, slovenly family is routinely abused and vilified by their father - the corrupt local politician Bill Heslop (Bill Hunter). Frustrated with his family's lack of ambition or drive he has turned into a tyrant.While at dinner with some property developers, Heslop runs into his mistress who has done well in a cosmetics pyramid marketing scheme who recruits Muriel into the scheme. The following day Muriel's mother writes a blank cheque to cash intending for the money to let Muriel buy into the scheme.
Escalating from petty theft, Muriel uses the blank cheque to defraud her parents, draining their bank account of $12,000. Muriel uses this money to stalk her former friends on Hibiscus Island.While on the Island, Muriel's former friends confront her over her stalking, requesting that she leave them alone. Later in the evening, Muriel runs into Rhonda Epinstock (Rachel Griffiths). Rhonda confronts Muriel's former social-group who bullied her in high-school. Rhonda and Muriel catch up, with Muriel inventing grandiose fantasies of a fiancé. Later, Rhonda and Muriel perform an Abba lip-sync in the talent contest. It is unclear where they got their elaborate costumes.At the end of her holiday, Muriel returns home to be confronted by her mother over the fraud, however Muriel immediately moves to Sydney where she shares a flat with Rhonda and changes her name to Mariel.
One night, Rhonda and Mariel go to a nightclub. Rhonda meets two American sailors while Muriel takes a nice man, Brice Nobes, she met at her minimum wage job in a video store. They return separately to the small flat that Mariel and Rhonda share. While engaged in rough foreplay, Mariel and her companion break a window. The sailors that were in Rhonda's bedroom go out to investigate and assume that Mariel's companion is attempting to rape her. It is during this episode that Rhonda falls down and apparently paralysed.While waiting overnight in the only empty emergency room in Sydney, Mariel calls her family home where she learns that the Australian Federal Police are investigating her father for corruption.
Rhonda discovers she has cancer pressing on her spine and requires urgent surgery. Mariel then co-opts Rhonda's health crisis as the basis of a deception to obtain a free photo shoot from a Bridal shop. During one of Rhonda's rehab sessions, Mariel promises that she will take care of Rhonda and that they will never need to return to their hometown. Rhonda discovers that Mariel has tried on every wedding dress in Sydney and confronts her. Mariel confesses the depth of her deceptions.Bill and Mariel go to lunch with Bill's solicitors, where Bill confronts Mariel over her criminal fraud and Mariel confronts Bill about his infidelity.Rhonda's tumour returns necessitating more severe surgery making her paralysis permanent. No longer wanting to be bothered with her only true friend's disability, Mariel moves out, leaving Rhonda no option but to ask her mother to take her back to their Porpoise Spit. Mariel enters into a criminal conspiracy to commit visa fraud marrying South African swimmer David Van Arkle so that he can stay in Australia. Mariel agrees to marry David for $10,000.At Marial's elaborate wedding, her former friend group serve as the bridesmaids, while Rhonda bound to her wheelchair is relegated to being a guest, which leads to an awkward discussion between Rhonda and her former bullies. Mariel's father is accompanied by his mistress and Mariel's mother arrives late to the wedding, missing the actual ceremony.At the end of the ceremony, Mariel doesn't notice her mother at the back of the church and just walks past. After the ceremony Marial and David return to David's apartment who gives her a perfunctory tour of place. After showing her bedroom to Marial, David makes his contempt for Mariel known asking "what kind of person marries someone they don't know?" to which she replies "you did". He shows her a grudging respect when she says that she too wants to win. Realising that their marriage will be platonic, Mariel sits on her bed and contemplates the view.Meanwhile, back in Porpoise Spit, Mariel's mother accidentally shot-lifts a pair of sandals from a supermarket. Bill arranges with the police for the charges to disappear and takes Mariel's mother home where he announces his intention to divorce Betty and marry his mistress, Deidre Chambers.Betty commits suicide, although bill calls in favours to have it recorded as a heart-attack. Realising that Betty lived only for her family and that her constant marginalisation broke Betty's will to live, Mariel breaks down and realises that all the frauds, falsehoods and lies only lead to pain. David comforts Mariel and they consummate their marriage.In her mourning Mariel realises that she can move beyond her family-of-origin issues, that her actions have consequences and that her choices matter. The next morning Mariel asks David for a divorce, who has decided that he likes having her around. However, she leaves him at the hotel wishing him good luck for in the games.Bill asks Muriel to stay and help raise the kids; she refuses and returns to Sydney. She repays $5,000 of the $12,000 she stole, saying that she will repay the rest when she gets a job back in Sydney.Mariel visits Rhonda at her mother's house and offers to take her back Sydney. Rhonda accepts.

Question: Who is Bill's mistress?
Answer: Deidre Chambers