REESE WITHERSPOON started drinking to cope with the stress of her parents’ bitter battle in divorce court last year, which included charges that her dad was a bigamist, say sources!

That’s the heartbreaking secret behind the “Legally Blonde” star’s disorderly conduct arrest in Atlanta on April 19, after her husband, Jim Toth, was busted for DUI.

“Reese struggled with the vicious family feud between her mother and father throughout 2012,” said a source close to the “Water for Elephants” actress.

“Her mother was accusing her father of bigamy and it sent Reese into a tailspin.”

John and Betty Witherspoon separated in 1996, but never officially divorced. In January 2012, John married Tricianne Taylor, then claimed he couldn’t remember tying the knot!

Betty believed her then-70-year-old hus­band was suffering from early onset dementia and that the marriage was a plot by his new bride to get in his will.

Reese and husband Jim attended an emergency hearing at a Tennessee court to decide whether her father should be placed in a conservatorship, which means he would be deemed legally unable to take care of himself.

“This is a daughter who wants to help her parents, particularly her father,” explained their family attorney, Andra Hedrick.

Reese and Jim, 42, were in Georgia in April while the actress was filming “The Good Lie.” At 12:40 a.m. on the 19th, Officer J. Pyland noticed the couple’s rented vehicle weaving across the marked lanes of Peachtree Road. After pulling them over, Pyland says Reese, 37, disobeyed orders to stay in the car while he performed a field sobriety test on Jim.

After she refused to return to the vehicle, she was handcuffed and arrested. She then said: “Do you know my name? You’re about to find out who I am.”

Reese issued a public apology the following day.

This isn’t the first time the mom of three has been overwhelmed by family problems.

In 2002, her brother John was charged with aggravated burglary and sexual battery in Nashville after he allegedly entered a neighbor’s home and tried to undress her as she slept.

The actress has two children – daughter Ava, 13, and son Deacon, 9 – with ex-husband Ryan Phillippe. They divorced in 2007, and she mar­ried agent Toth in 2011. She gave birth to a baby boy, Tennessee, in September.

“Reese feels she has the weight of the world on her shoulders, and she started drinking alcohol  as a release,” added the source.