A Rhode Island man was convicted Wednesday in the murders of a brother and sister in their Brockton home in January 2010, officials said.

A Plymouth Superior Court jury convicted John V. Monteiro, 31, of two counts of first-degree murder, as well as home invasion and weapons charges, in the shooting deaths of Maria DePina, 26, and Jorge DePina, 30, District Attorney Timothy J. Cruz’s office said in a statement.

Monteiro was given a mandatory sentence of life without parole. His case will automatically be reviewed on appeal by the state’s highest court.

His lawyer, Barry P. Wilson, lamented the verdict in a phone interview.

“I thought the verdict was against the weight of the evidence,’’ Wilson said. “I don’t think that the Commonwealth proved its case beyond a reasonable doubt.’’

Prosecutors said that Jorge DePina owed Monteiro a debt, and that the defendant forced his way into the siblings’ Nilsson Street apartment on the morning of Jan. 21, 2010, then shot them both in the head.

Monteiro shot Jorge DePina three times as he slept, Cruz said in the statement. Maria DePina heard the blasts as she was getting ready for work and encountered Monteiro in a hallway. He shot her in the back of the head as she tried to flee, Cruz said.

“This was a brutal crime that decimated a family and stunned the Brockton community,’’ Cruz said. “An extremely dangerous man is now off of the streets and we have hopefully gained some justice for the DePina family.’’

Prosecutors said that after the killings, Monteiro sold the murder weapon to an informant working with the FBI, an allegation Wilson did not contest.

“Yeah, he did that. He’s a gun dealer,’’ Wilson said, while insisting there was insufficient evidence to convict Monteiro of the murders.

In 2012, Monteiro was indicted on charges of murdering another man, Gregory Goncalves Jr., 28, in July 2007. Cruz’s spokeswoman said prosecutors later dropped that case because two key witnesses were unavailable for trial.

Travis Andersen can be reached at travis.andersen@globe.com. Follow him on Twitter @TAGlobe.