From the top of the basement stairs he says he watched Phoenix Sinclair's final moments.


Hands trembling and looking down, Karl Wesley McKay's younger son shared with the jury what he saw the day phoenix died.


The boy, now 15-years-old but 12-years-old at the time, says McKay and his common law wife Samantha Kematch beat Phoenix for about 20 minutes, passing her back and forth.


He says his father punched the girl in the head, pushed her, until she fell on the basement floor.


"I didn't hear her crying. I said what's wrong, I poked her. I just thought she was playing dead. I touched her back and it was all cold. I put my hand on her mouth and she wasn't breathing. I thought, oh my god."


Later that night the teen says McKay and Kematch wrapped the girl up in a tarp, put her in the back of the trunk, and told him to watch over his baby sister because they were going to the dump to bury Phoenix.


The teen says he was told not to tell anyone, and that if anyone did ask, to say Phoenix moved to Winnipeg with her dad.


He kept the secret. He said he was afraid, after witnessing months of abuse...


The boy described how McKay would beat Phoenix with a pole, fridge handle and a metal broom stick.


He said McKay hit Phoenix so hard the broom stick broke.


Then McKay used the other half to hit Phoenix's knuckles until they bled.


Kematch's lawyer argued her client sometimes tried to stop McKay from beating Phoenix by taking her away to another room.


The defense also insisted Kematch only used a weapon on Phoenix once, and never hit her as hard as McKay did.


McKay's legal team argued how Kematch would hit the girl when McKay was out, insisting the mother could have left the house to get help or call 911 at anytime.


With a report from CTV's Stacey Ashley