Danielle Cassin, 27, told relatives Levi-Blu Cassin had been unresponsive and not breathing on the morning of his death in February 2013.
Birmingham Crown Court previously heard Levi had died from internal bleeding caused as a result of violence.
His parents Ms Cassin and Mark Piper, 31, deny murder.
Danielle Cassin's half-sister, Kirsty Cassin, told the court they met at hospital after Levi was pronounced dead.
Danielle Cassin told her she had gone to check on Levi and found he was "not breathing and cold to the touch".
Kirsty Cassin also told the jury her half-sister was known to use heroin.
Her behaviour became "erratic" after the defendant rekindled her relationship with co-accused Mr Piper in October 2012, she said.
The court heard police were called in January 2013 after a neighbour witnessed Mr Piper hit Danielle Cassin.
Daniel Cleal, who lived nearby in Chelmsley Wood, West Midlands, said he saw them arguing in their kitchen, a bedroom and the living room.
He said he saw Mr Piper put his hands around Danielle Cassin's neck.
"I saw [him] punch Danielle and she fell to the floor," he said.
Mr Cleal, who said he only knew both defendants by sight, told the court he did not know if Levi had been present during the argument.
Danielle Cassin and Mr Piper, of Nightingale Avenue, Chelmsley Wood, also deny causing or allowing the death of a child, assault, neglect, abandoning a child and causing unnecessary suffering to a child.
The trial continues.
A mother accused of murdering her 22-month old son told her family she awoke to find his "cold and staring" body, a court has heard.