JENNIFER ANISTON isn’t the only one in her family whose biologi­cal clock is ticking – her 80-year-old actor dad John Aniston is anxious for a grandchild!

But Jen’s half brother Alex, known as AJ to pals, may already be mar­ried and, in a pregnancy bombshell, might beat his 45-year-old sister to the maternity ward!

Punked-out AJ’s live-in love, pink-haired Adriane Hallek, recently walked out of a Los Angeles medical clinic clutch­ing an ultrasound.

What’s more, wild-child Adriane, a tattooed makeup artist and model, announced on her Facebook page on Jan. 29 that she’d “married AJ Indavan” – the name that AJ uses on the social network. When The ENQUIRER tracked down Adriane to ask about AJ, she said: “He’s a great guy, but I don’t want to talk about him. He likes his privacy.”

Adriane also revealed that she hasn’t met former “Friends” star Jen, but clammed up when asked if she’d tied the knot with AJ.

“I don’t want to answer anymore questions,” said Adriane, who goes by the nickname “Gig Gles.”

A source close to the couple added: “When I saw that Adriane posted they’d gotten married, I was surprised. But knowing her, I could see them being crazy enough to do it.

“Adriane moved to Santa Cruz to be with AJ, and I heard they’re living in a house in the woods. They seem like a really good couple and they look happy together.”

As The ENQUIRER has revealed, 24-year-old AJ – who’s the son of Jen’s daytime soap star dad and his second wife, Sherry Rooney – often slept in the back of his van while drifting between L.A. and Alaska, hence his Facebook moniker, AJ “InDaVan.”

We published world-exclusive photos of AJ’s alternative lifestyle, which included attending the in­famously drug- and booze-soaked 2013 Burning Man festival in the Nevada desert.

At the time, AJ sported full punk mode, complete with a spiky hair­cut, patched-up shorts, scruffy work boots and a huge “Live Free” tattoo emblazoned across his stomach.

While her half brother may already be married, “We’re the Millers” star Jen has yet to walk the aisle with fiance Justin Theroux even though they’ve been engaged since August 2012.

“Jen’s dad John longs for grand­children and knows that if she doesn’t hurry up and get hitched, he might not live long enough to see her kids,” said a source. The elder Aniston, who plays Victor Kiriakis on “Days of Our Lives,” will turn 81 in July and has suffered from health issues in the past.

“John knows he’s not getting any younger and he wants to meet his first grandchild,” added the source.

“He’s aware how deeply Jen’s di­vorce from Brad Pitt affected her, and he sees how happy Justin has made her. So like any loving dad, he wants to live long enough to see his daughter married with a family.”

Meanwhile, Jen’s 77-year-old mother Nancy Dow, who suffered a stroke in September 2011, is also said to be anxious for Jen to start a family. The screen beauty and her mom have endured a stormy relationship, but Jen and her father have been on good terms for years, even though our source says he has some­times questioned her choice of beaus.

“Although John hasn’t always loved the guys Jen dated, he’s given his stamp of approval to Justin,” noted the source.

But while he’s waiting for Jen to finally marry Justin, John could get some unexpected news from AJ.

“It would be ironic if AJ makes his dad a grandfather before his sister Jennifer does,” said the close source. “I don’t think anyone is expecting that!”