Argueta cannot hold back his tears.
He's speaking out, urging his sister's killer to surrender to police.
He has few words to describe the kind of monster that would gun down a mother and her baby boy.
"That person who ever did it, did not have a heart," said Argueta, "Especially like how they did it you know, just threw them in the woods, like an object, you don't do that, not even to your worst enemy."
Last Friday morning, the bodies of Vanessa Argueta and 14-month-old Diego were discovered in a wooded area behind a warehouse in Central Islip.
Monday, the family is so despondent, they stand before a shrine dedicated to the two, who were last seen on Thursday.
Argueta had no idea his sister had been murdered until police tracked his mother down through a cell phone found at the crime scene.
Argueta says his sister, Vanessa, was a devoted mother.
She and her son were inseparable.
"My sister was a very loving and caring person. Just like in all the pictures they have, she would always call him her little prince," said Argueta.
Police are still searching for the gunman.
Sources close to the case say investigators have a few strong leads, and the search spans two counties.
That's because Argueta lived in Hempstead, and was found in Central Islip.
For the victims' family, there is so much sadness, and there is little they can do but wait.
"It's a lot of pain, it's never gonna cure. For me they still alive, in my heart they're alive," said Argueta.