A Long Island man who has pleaded guilty to terrorism charges had no social skills as a child, and was bullied in school and shunned by his drug-addicted father, family members testified Thursday in a federal courtroom.

When he was younger, Justin Kaliebe, now 21, struggled to fit in, his older sister said in U.S. District Court in Central Islip.

“He was known as the ‘weird kid’ from the beginning, and children treated him that way,” Jami Kaliebe, 25, of Bellville, New Jersey, testified at a pre-sentencing hearing in Central Islip. “I never really knew him to have ... a true friend.”

Before Justin Kaliebe could speak coherently, when he was about 7, she said she had to serve as his translator.

Kaliebe’s mother and former pediatrician also testified at the hearing in which defense attorney Anthony La Pinta of Hauppauge presented possible mitigating evidence.

Kaliebe, of Bay Shore and Babylon, faces up to 30 years in prison.

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He was arrested at Kennedy Airport in January 2013 as he was about to board a flight on his way to Yemen. In February 2013, he pleaded guilty to attempting to provide material support to terrorists. He told a federal judge that he had decided as early as 2011 to go to the Middle East to join a militant offshoot of al-Qaida.

But in court Thursday, Debra Livoti, of Baylon Cove, described her son as a “beautifully good-natured child” who struggled to engage people because he rarely made eye contact.

Jami Kaliebe testified that after her parents split up, she and Justin were shuttled between her mother’s “beautiful” home with her new husband and their father’s “negative, violent, addicted household” in Brentwood.

Kaliebe, who didn’t get along with his stepfather, later moved in with his father in Brentwood, but was forced to sleep on the floor, the sister said.

In Brentwood, however, he converted to Islam and got involved with a mosque, where he “felt embraced,” she said.

During cross-examination by Assistant U. S. Attorney Seth DuCharme, Kaliebe’s mother and sister acknowledged they didn’t know he was reading terrorist propaganda or planning to travel to Yemen.

His former pediatrician, Jason Hitner, said Kaliebe was born with low testosterone and suffered from developmental delays.

Hitner said Kaliebe exhibited symptoms of Asperger’s syndrome, a disorder characterized by difficulties in social interaction, and some level of autism.

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During questioning by prosecutors, Hitner said he couldn’t find a mention of autism or Asperger’s in his medical notes.

Testimony in the hearing is expected to conclude Friday.