Joe Biden's son Hunter Biden's affair with late brother's widow exposed after daughters read his texts

Hunter Biden's relationship with Hallie Biden, the widow of his elder brother Beau Biden, first emerged publicly in 2017. Photo / AP

Hunter Biden's daughters found out he was having an affair with his late brother's widow after reading his text messages.

Details of the shocking discovery were revealed in a new book by Hunter's ex-wife, which is due to be published next week.

It is expected to cause further embarrassment for Joe Biden following a host of previous revelations about his son's drug-taking and business dealings.

Kathleen Buhle married Hunter Biden in 1993 and they have three daughters, Naomi, 28, Finnegan, 23, and Maisy, 21.

In 2017, it emerged publicly that Hunter was in a relationship with Hallie Biden, the widow of his elder brother Beau Biden, who died from brain cancer in May 2015.

Joe Biden publicly backed the relationship, saying he was "happy" that Hunter and Hallie "found each other".

In her book, If We Break, Buhle describes how, following the death of Beau Biden, Hunter began spending most of his time at Hallie's home.

A therapist told Buhle it was important for her husband to do so as part of the grieving process.

Hunter had already admitted cheating with prostitutes while away on business trips, Buhle wrote.

In the summer of 2015 she found a crack pipe in an ashtray, and threw her husband out of their house.

Then, in November 2016, her daughter Finnegan telephoned her from the home of the family therapist, and asked her to go there.

When Buhle arrived, Finnegan was curled up in a chair weeping, and Naomi was on a speakerphone.

Finnegan asked the therapist to tell their mother what was going on, saying: "We can't do it."

Hunter Biden walks with his then-wife Kathleen, along with the then US Vice President Joe Biden and Jill Biden at the funeral service for Senator Edward Kennedy in 2009. Photo / AP

The therapist told Buhle: "Kathleen, Hunter's having an affair with Hallie."

Her daughters then told her they knew because they had found their father's phone, and discovered text messages between him and their aunt.

They had already telephoned their father, and told him that they knew about the affair.

Buhle wrote: "'Oh my God'. This was all I said. Was this what shock felt like? I could see Finnegan's face relaxing now that the secret was out and I hadn't fallen apart.

Hunter Biden's relationship with Hallie Biden, the widow of his elder brother Beau Biden, first emerged publicly in 2017. Photo / AP

"If anything, I felt a strange vindication. Not only had I not been crazy, but it was so much worse than I could have imagined. I was shocked, but not heartbroken."

Her divorce from Hunter was finalised in 2017.

Buhle described Joe Biden as the "sun around which we all revolved", and how he would always introduce her as his daughter.

But a "sore point" for her was that, unlike Hunter and their daughters, she was not provided with Secret Service protection when Biden became vice president following the 2008 election.

She said it made her feel she was "not truly a Biden," and was "another reminder that I was not 'family'."