Bernadette Clarke said her sister's nine-year-old son had been left with no mother.
"The man shot her at point-blank. Left the family broken," she said.
"How long is this going to go on for?".
The first-class constable faced a three-week trial in the WA Supreme Court over the September 2019 killing of JC.
After deliberating for three hours on Friday the jury found the officer, who cannot be named for legal reasons, not guilty of JC's murder.
The accused officer wept in the dock as the jury also returned a not guilty verdict to the lesser charge of manslaughter.
Family supporter and Noongar woman Megan Krakouer said there was still "no equality" for Indigenous people, 30 years after the Royal Commission into Aboriginal Deaths in Custody.
"This is what we have to deal with in the state of Western Australia. This happens far too many times and it hurts, and it's wrong," she said.
"No conviction ... and you wonder why Aboriginal people are so angry and disillusioned with the police and the system."
WA Police Commissioner Chris Dawson said the police force is continuing to work on improving engagement with the Indigenous community.
"This tragedy is one of the most difficult chapters in the history between Aboriginal people in Western Australia and the WA Police Force," he said on Friday.
"We have been working very hard over the last two years to build on what has been a very long history of engagement between police and Aboriginal people since colonisation.
"We will continue to build our strong relationship with Aboriginal people, accepting there are strong emotions running out of this tragedy.
"But that won't stop us. We will continue to walk in step with Aboriginal people."
Commissioner Dawson said the officer who has been acquitted has been stood down over the past two years and discussions will be taking place around his future.
There will be a coronial inquest into JC's death.
Additional reporting: AAP