The Bronx teen who watched his mom get fatally shot through their apartment peephole narrowly escaped her deadly fate — while the other son who triggered the shooting is devastated over their mother’s death, kin told The Post on Monday.

Alexander Angulo, 15, was helping mom, Wendy Martinez, 45, clean their Concourse Village apartment Saturday evening when someone rang their doorbell. The helpful son offered to answer, but his mom stepped in instead, according to Kimberly Martinez, Wendy’s niece and Angulo’s cousin.

Moments later, Wendy was dead with a bullet through her head — the victim of what cops suspect was retribution for her older son’s alleged involvement in a gang-related killing.

“My youngest cousin [Alexander] was home — he was cleaning when he heard a knock,” Kimberly told The Post. “He was going to get the door, but [his mom Wendy] said, ‘No, I’ll get it.’ When she went to get the door, she looked through the peephole, and they shot.”

Angulo “was there, and he saw her fall to the ground,” she said.

The teen called 911 and hid in a closet until police came, officials have said.

Investigators believe the grisly slaying was connected to Wendy’s older son, Brian Solano, 25, who is on trial for murder after allegedly killing a gangbanger’s brother in 2014. Police sources called the weekend killing of Solano’s mom “street justice.”

Solano, who lived with his mother and brother, is torn up over his mom’s death, Kimberly said.

“He’s broken. Everybody’s broken,” she said.

The younger brother, Alex, is “trying to hold it together,” Kimberly added.

“That was his rock. It’s traumatizing, but he is actually very strong,” she said.

The death came just days before Wendy’s 46th birthday Tuesday, according to the niece, who set up a GoFundMe to raise money for the family.

“She was like a mom to me. She was so humble and sincere, she always smiled,” Kimberly said. “I hope she didn’t suffer at all.”

Solano had a hearing scheduled Monday in the 2014 murder case, but a judge postponed it following his mother’s murder.

Solano’s private investigator, Manny Gomez, got two grand-jury witnesses to recant their testimony and the hearing was to “determine whether there was misconduct in the recantations,” according to a Bronx DA rep.