MIRANDA LAMBERT and hubby Blake Shelton joke about their hard-partying lifestyle, but boozing is no laughing matter to her younger brother – he served time behind bars for drunk driving!

According to court records obtained by The ENQUIRER, Leslie Adams Lambert, now 25 – who goes by Luke – was arrested for DUI in 2010 near his home in Austin, Texas. His blood alcohol content was .144, nearly double the state’s legal limit. Luke also admitted to mixing the booze with the anti-anxiety drug Lexapro, which can cause drowsiness and dizziness. Luke served 12 days in jail, lost his license for 90 days and was ordered to pay court costs after pleading “no contest.”

Despite the dark family history, Miranda, 30, continues to boast about her love of the bottle – without ever mentioning her brother’s secret.

“Both Miranda and Blake put themselves out there as these hard partiers who revel in the high life and brag about how much they drink,” a source said. “Little do people know it’s been a problem for one of their family members.”

As The ENQUIRER revealed last week, Miranda lost 30 pounds and looked stunning at the CMA awards. But she didn’t cut out booze to help drop the pounds – she just switched from beer to the “Mirandarita,” a custom concoction of rum, raspberry lemonade and Sprite Zero – saying it’s “practically a health drink.”

Meanwhile, Blake’s heavy drinking is the long-running subject of jokes on “The Voice,” the hit NBC singing competition where the 37-year-old country hunk serves as a coach. He also posts about his boozing on Twitter. And earlier this year, Blake scoffed at reports that his wife was sending him to rehab, saying: “Incorrect. She sent me to the liquor store!”

But not everyone thinks it’s funny.

“This kind of drinking horseplay between Miranda and Blake is irresponsible at best, reprehensible at worst,” blasted another source. “Luke’s arrest is no joking matter and Miranda knows it.

“That’s why she’s kept it under wraps. People would be outraged. Her brother could have killed himself – or someone else.

“Miranda could have done a lot of good by talking about it because there are kids who hang on her every word. Instead, she’s kept it secret.”