Dzhokhar Tsarnaev’s defense team today called Tamerlan Tsarnaev’s mother-in-law to the stand in the first appearance by any family member of the Boston Marathon bomber and his late co-conspirator brother.

Judith Russell testified that for Tamerlan, Islam, “became a sort of, I don’t know, obsession.”

The defense said in their opening statement today that they would paint a portrait of Tamerlan as “obsessed with jihad.”

Russell is mother of Katherine “Katie” Russell, Tamerlan’s widow. Their daughter is now 4 1/2.

Russell, who had never publicly spoken since the bombings, described Tamerlan as transforming into a radical, growing his hair and beard, and said her daughter “was really cut off from a lot of things she was involved in before.”

Katherine Russell “started to learn about Islam,” her mother said. “That was a change. She hadn’t been religious as a child.” She owned the book “Islam for Dummies.”

Two years after the bombings, Katherine Russell — who dropped out of Suffolk University in her junior year to marry Tamerlan when she became pregnant — is still a practicing Muslim.

Russell said she vocally opposed Tamerlan’s “selfish” solo trip to Russia in 2012, which lasted six months, and during which, according to the defense, he became radicalized.

And, the mother-in-law said, she found Tamerlan’s constant blather about politics “annoying.” Although “he had a certain charm,” she said, “I was not pulled in.”

She was happy the couple took a break after Tamerlan cheated on Katie, but Russell lamented she couldn’t keep them apart.

Three days after the bombing, the FBI released photos of their bombing suspects.

“I saw the pictures. I’d never met Dzhokhar and I didn’t think it was him (Tamerlan),” Russell testified, beginning to weep.

That night, before authorities had put names to their faces, the Tsarnaevs fled in a bloody crime spree, killing MIT officer Sean Collier and engaging in a violent Watertown showdown that left MBTA officer Dic Donahue wounded.

These days, Russell said, her daughter “is healing from this experience. She is getting her life together and is lighter in spirit and more like the Katie we knew.”

The defense team today began the fight to save Dzhokhar’s life today by showing jurors a bleak, desolate photo of what he’s willing to endure over death: the Supermax federal prison for terrorists in Florence, Colo.

“He was a good kid,” said anti-death penalty attorney David I. Bruck said in his 47-minute opening statement. “His last chance came when he was 19, and he will never be given another.”

In Supermax. Bruck assured jurors hearing from the defense for the first time in the sentencing trial, Tsarnaev, 21, will fade into oblivion — unable to do interviews, write his autobiography or enjoy so much as “a view of the outside world.”

“We have now seen more pain, more horror, more grief in this courtroom than any of you would have ever thought possible,” Bruck said. “Whether you vote for death is up to each one of you, but the man who conceived, planned and led this crime is beyond our time.”

Bruck was referring to Tamerlan, 26, who Dzhokhar ran down and killed three days after the April 15, 2013, terrorist attack.

Tamerlan’s influence over Dzhokhar, raising him in the vacuum left by their mentally ill parents, is expected to take center stage in testimony all week week.

“Tamerlan had power over Dzhokhar,” Bruck told the jury.

And Dzhokhar, he said, “was a lost teenager with very little motivation to do anything on his own.”

Bruck said witnesses will include relatives of the Tsarnaev brothers from Russia, who will testify through an interpreter.

They will give context to “why Tamerlan set himself and his brother on this terrible course … Tamerlan was obsessed with jihad,” Bruck said. And they will tell the court “how fanatical and unreasoning Tamerlan seemed to be” during a visit abroad in 2012.

Witnesses also will include friends of Dzhokhar from the University of Massachusetts Dartmouth.

And at least one expert in the neurological development of teens will testify — describing, Bruck said, that adolescents are like cars “with really powerful engines and faulty brakes.”

The Herald reported today that such testimony has been key in capital cases involving teens since the Supreme Court’s landmark decision in Roper v. Simmons, which outlawed the death penalty for defendants under 18.

In its decision, the court cited research that has found the brains of young adults undergo development throughout late adolescence. Some experts say that development continues well into the 20s.

Dzhokhar was 19 when he and Tamerlan committed the bombings.

DEVELOPING…